docs: Phase 1 architecture spec — ADRs 001..006, spec docs, OQ promotion

The Phase 0 OQ ledger (OQ-TN-01..10) promoted into
docs/architecture/open-questions.md (now OQ-TN-01..14, with four
new Phase 1 residues). Six ADRs and four spec docs, all Draft, all
decision-shaped per the SDD process (ADRs carry the WHY; specs carry
the WHAT and reference by number):

ADRs:
- 001 open-params-layout: params = {resource, substrate} — the
  producer's stable name, not an address; wire-stable from the first
  consumer; unknown substrates fail loudly at schema time
- 002 alpn-strategy: single alk/tunnel ALPN (option A); substrate in
  params selects the framing; prior art (SSH/SOCKS5/udpgw) gives no
  reason for the split
- 003 codec-and-udp-framing: raw pass-through (stream) / mandatory
  [len: u16 BE] (UDP) — F-2 mandate recorded (the codec is mandatory
  for correctness, not cosmetics); len=0 = legal empty datagram;
  OQ-TN-13 resolved fail-loud (truncation)
- 004 no-backend-trait: halves functions at the assembly layer;
  listen is an establisher shape; hub re-produce deferred(OQ-TN-12);
  trait re-evaluated at the alknet ADR-078 convergence threshold
- 005 consumer-session-owns-teardown: TunnelSession with
  open/adopt/stream_halves/take_halves/pump_against/close/join/Drop —
  the W3 adopter gap closes structurally; Drop is the best-effort
  fallback
- 006 access-control-posture: the open gate is the boundary;
  TUNNEL_OPEN_SCOPE = tunnel:open (stable once published); op-level
  ACL; ownership seam; no allowlists in v1; identity = 0.7.0's
  precedence chain

Spec docs:
- overview.md: purpose, resource model, deps (alkcall 0.7.0,
  wasm-clean default, local feature), module map
- wire.md: the open op (params/reply/typed errors), the data plane
  by substrate, sentinels + half-close, byte diagrams
- producer.md: spec, establisher (dial + listen shapes), pump
  handler (pump_bidi inline, R-02), registration API, ACL posture
- consumer.md: TunnelSession (forward open + reverse adopt
  construction), data plane, teardown API (close/join/Drop incl.
  pump-less join semantics)
- bast.md: the BAST doc for the UDP codec (convention 12's trigger
  fired — the framing IS binary beyond pass-through)
- open-questions.md: OQ-TN-01..14; 01..10 promoted (faithful to the
  phase-0 ledger's final states), 11 partially resolved (collision
  domain = per-producer registry per ADR-001; lifecycle open),
  12 deferred(scope), 13 resolved fail-loud (ADR-003), 14 open

Verified by an architecture-reviewer pass (2 criticals, 6 majors,
10 minors — all fixed: ADR-001/003-vs-OQ decision-state contradictions
resolved; the Layer-2 mislabel corrected to the ADR-047 §4 per-session
fork; alktty/alknet ADR misattributions fixed; codec placement pinned
to the establisher (pump stays substrate-agnostic); reverse-path
construction named (TunnelSession::adopt + pump_against); security
posture promoted to ADR-006; BAST doc written; F-2 rationale deduped;
README tables completed; channel_id > 0; MTU wording fixed; ADR-047 §5
attributions corrected; impacts lines added to unresolved OQs; review/
ledger pointer paths added to README references).

Verification: doc set internal cross-refs all resolve; cargo test,
fmt --check, doc --no-deps clean

docs/architecture/decisions/001-open-params-layout.md

@@ -0,0 +1,120 @@
+# ADR-001: Open-Op Params Layout — Resource + Substrate
+
+## Status
+
+Accepted (2026-09-07)
+
+## Context
+
+The tunnel open op's `params` (the `input` of the
+`channels/tunnel/sub` operation, alkcall ADR-047) is the wire-stable
+carrier of "what to open." alknet ADR-071 §ALPN table noted
+`alknet/tunnel` as `[0, 1]` data in/out only; the addressing scheme
+was never decided there. Phase 0 resolved the framing of the problem
+(the OQ-TN-01 reframe, 2026-09-05): the rich "remote addressing"
+framing was an XY problem — except in the `-D`/dynamic case (which
+composes at the assembly layer), a tunnel is either TCP or UDP, and
+`params` need only *identify a produced resource*, not carry a
+general-purpose address.
+
+Constraints that shaped the decision:
+
+- `params` is ALPN-specific JSON, interpreted by the open op's
+  establisher/handler — never by the channels layer (alkcall
+  ADR-039). The layout is a one-way door once a consumer exists
+  (wire-stable once published; additive changes only).
+- The identifier must be substrate-extensible without format changes
+  (a new substrate is an additive `substrate` value, not a v2
+  format). SSH's `direct-streamlocal` (same open-op shape, degenerate
+  address slots, new type string) and SOCKS5's ATYP (scheme-tagged
+  addresses) are the prior-art anchors; both support the "one
+  identifier + one discriminator" reduction.
+- The producer owns the backing: the consumer never learns an
+  address. A `host:port` in params would leak the producer's
+  topology into the wire and pin the wire to address-shaped targets.
+- alkcall's registry schema-validates the open op's input
+  (`input_schema` runs before the establisher — review 007 Part B),
+  so the layout must be schema-describable (no per-substrate
+  polymorphic payloads in v1).
+- Both POCs used exactly this shape (`TunnelParams {resource,
+  substrate}`); 17 + 16 tests rode it end-to-end.
+
+## Decision
+
+`params` for the `channels/tunnel/sub` open op is a self-contained
+JSON object:
+
+```json
+{
+  "resource": "postgres-primary",
+  "substrate": "tcp"
+}
+```
+
+- **`resource`** (string, required) — the produced resource
+  identifier: the producer's stable name for the tunnel target. NOT
+  an address; the producer's registry (an assembly-side construct,
+  OQ-TN-11) maps the name to its backing (a local port, a docker
+  container's port, an in-process service, a unix socket path).
+- **`substrate`** (string, required) — the extensible discriminator:
+  `"tcp"` | `"udp"` | `"unix"` in v1. A new substrate is a new value,
+  not a format change — the same additive property SSH gets from
+  channel-type strings and SOCKS5 gets from ATYP values. The
+  discriminator also selects the data-plane framing (ADR-003): the
+  establisher and both pumps must agree on it, so it rides the open
+  op rather than being inferred.
+- The open op's `input_schema` pins both fields as required strings;
+  `substrate` is enum-validated against the v1 set. Unknown
+  substrate values are a schema failure (typed open error) — a
+  consumer speaking a newer substrate to an older producer fails
+  loudly, not silently.
+- Substrate-specific detail (the path of a unix socket, the UDP
+  associate semantics) is owned by the producer's registry, not the
+  wire. The resource name is opaque to the protocol.
+- The open-failure error path rides alkcall ADR-049 (typed
+  `channel:open_failed` with `details.reason` ∈ `dial_failed` /
+  `unknown_resource` / `resource_shortage` / `handler_error` /
+  `timeout`): an unknown resource name is `unknown_resource` — no
+  params-level error field, no establishment frame (the pre-ADR-049
+  OQ-TN-09 hunch is superseded).
+
+Rich in-band addressing (SOCKS5 ATYP, per-datagram remote addresses)
+enters only through the `-D`/dynamic-target composition path — it is
+carried INSIDE the tunnel payload by whatever protocol the tunnel
+carries (socks5, udpgw-style framing), never in base params.
+
+## Consequences
+
+- **Wire-stable from the first consumer.** Both fields are required;
+  renames are a breaking wire change (do not). Adding an optional
+  field is additive-safe; adding a `substrate` value is
+  consumer-progressive (an older producer rejects with
+  `invalid_input`-class schema error — the SSH "unknown channel type"
+  posture, loud not silent).
+- **The BAST question is moot for params.** Params is JSON inside the
+  channels open op (which is already a JSON envelope); the data
+  plane's binary framing (ADR-003) carries its BAST doc at
+  `docs/architecture/bast.md`. Per AGENTS.md convention 12, the
+  hand-rolled codec remains the runtime implementation with the BAST
+  doc as the contract.
+- **Producers own a naming discipline.** Resource ids are
+  producer-scoped names (OQ-TN-11: collision domain = the producer's
+  registry). Consumers learn available resources through the
+  ACL-filtered ops listing (OQ-TN-08) — the spec's
+  `OperationSpec.description` SHOULD carry a human-readable hint
+  (e.g. "postgres on the worker's tailnet") since it round-trips
+  through discovery.
+- **No per-connection dynamic targets in base params.** `-D`/SOCKS
+  composes by tunneling a socks5 connection (OQ-TN-08); a future
+  multi-endpoint UDP gateway resource would be self-describing
+  framing inside the channel payload, invisible to params.
+
+## References
+
+- Phase 0: OQ-TN-01 (the reframe + survey input),
+  `docs/research/ssh-socks5-survey.md` §OQ-TN-01 residue
+- alkcall ADR-047 (openable ALPNs are operations; `input_schema`),
+  ADR-049 (typed establishment errors)
+- Forward POC: `docs/research/poc-summary.md` (the shape riding 17
+  tests); reverse POC: `docs/research/reverse-poc-summary.md`
+- OQ-TN-01 (promoted), OQ-TN-11 (naming residue)