fix: Unit 6 — convention + doc cleanup (C-09, C-20-rem, C-22-rem, C-24, P-10, P-11)

Conventions satisfied, `cargo doc` clean, no actively-wrong comments,
producer/consumer naming consistent in the call/registry docs.

P-10 — `pump_sink` matched the string literals "call.published",
"call.completed", "call.aborted" (dispatch.rs) instead of the
EVENT_PUBLISHED / EVENT_COMPLETED / EVENT_ABORTED constants the rest
of the file imports. Replaced with the constants — pure refactor
hazard, no behavior change.

C-20 remainder — the wrong "SAFETY:" comment at from_call.rs:271
marked no `unsafe` block and was factually wrong (described a `'static`
return that isn't what `derive_alpn_from_op_name` does — it returns
`Option<String>`; the leak happens in `leak_alpn`). Reworded to a
plain note about the `'static` lifetime requirement. Also reworded
the abort-cancels claims in the `pump_sink` and `pump_stream` doc
comments (dispatch.rs): both claimed `call.aborted` "cancels the task
and drops the handler future" — the handler is actually `join!`-ed to
completion and not yet cancelled (the abort-cancels-Pub mechanism is
review 001 Unit 9). Trimmed step-numbered narration comments in
adapter.rs / client.rs that restated what the code does, keeping the
ordering-constraint and REQ-CH comments. The big reassembly.rs
deliberation landed with Unit 4; this finishes the remainder.

C-09 — fixed the 2 remaining `cargo doc` warnings (was 4; the
register_openable links were fixed in Unit 3):
- `unresolved link to default_policy` (operations.rs:50) — the
  [`default_policy`] intra-doc link resolves to
  super::policy::default_policy; used the full path.
- `env is both a module and a macro` (channels/mod.rs:30) — the
  [`env`] link collided with the std `env!` macro; qualified as
  [`self::env`].
`cargo doc --no-deps` now emits 0 warnings.

C-22 remainder — removed the filler `PhantomData` test at client.rs
(`let _ = std::marker::PhantomData::<ChannelClient>;` — asserts
nothing). The env.rs tautology was already removed in Unit 3.

C-24 — replaced "client→server streaming" with "producer→consumer
streaming" in call-protocol.md, operation-registry.md, README.md, and
open-questions.md (4 occurrences). Per AGENTS.md §8 the convention is
producer/consumer, not server/client. The remaining "client→server"
references in channels ADRs 034/037 are in stream_type table contexts
that Unit 10 (C-26) will handle as part of the spec-doc renumbering.

P-11 — amended ADR-046 §3's SinkHandler type so the stream item type
matches §6. §3 declared `Pin<Box<dyn Stream<Item = Value> + Send>>`;
§6 declared `Pin<Box<dyn Stream<Item = Result<Value, CallError>> +
Send>>`. The code uses §6's shape uniformly (registration.rs:32-40,
aliased as PublishStream). §3's text and the Door-type section are
amended to match §6; the Door-type section already marked the concrete
stream item type a two-way-door detail, so this is a text correction,
not a design change. Added an amendment note dated 2026-08-13.

Verification:
- cargo test --lib         → 449 passed, 0 failed (was 450; -1 removed filler test)
- cargo clippy --all-targets -- -D warnings → clean
- cargo fmt --check        → clean
- cargo doc --no-deps      → 0 warnings (was 2)

src/channels/mod.rs

@@ -27,7 +27,7 @@
 //!   (ADR-041, amended by ADR-047 §7 — opener ledger).
 //! - [`client`]: `ChannelClient` — transport-agnostic
 //!   `from_connection` (ADR-043).
-//! - [`env`]: `ChannelOperationEnv` extension trait (ADR-047 §4 —
+//! - [`self::env`]: `ChannelOperationEnv` extension trait (ADR-047 §4 —
 //!   keeps the call crate free of channels types).
 //!
 //! See `docs/architecture/` for the full specification.