ADR-007: RFC 7250 cert-type negotiation — the offer follows the identity (OQ-TLS-10 resolved)

Resolve the cert-type negotiation gap (review 001 §U-3, OQ-TLS-10) by
deviation from alknet: the gap was a defect in the prior art (alknet's
code never delivered its spec's raw-key-over-TCP promise — ADR-082
"works for both QUIC and TCP+TLS"), not behavior to preserve.

- FingerprintPinVerifier::requires_raw_public_keys() derives from the
  pin format: ed25519: -> true (offer [RawPublicKey]), SHA256: ->
  false (X.509 offer). Crate pin client now completes against the
  crate raw-key server; SHA256: pins negotiate unchanged.
- RawKeyClientCertResolver presents the SPKI under the X.509 offer
  (only_raw_public_keys() == false): a raw-only client offer can only
  negotiate against a requires_raw server verifier, and
  AcceptAnyCertVerifier correctly stays on the default (accepts both
  cert types). The server extracts the ed25519: fingerprint from the
  SPKI bytes either way.
- Fail-closed preserved and strengthened: an ed25519: pin against an
  X.509 server now aborts at negotiation (suite 2b), never a
  downgrade; no API change (no public signature affected; the fix is
  invisible to consumers apart from working handshakes).
- tests/handshake_behavior.rs: suite 3 now runs crate-native (no
  custom iroh-shaped verifier), new negotiation fail-closed suite,
  suite 3b inverted to end-to-end success; invariant_pins.rs
  resolver-offer assertions flipped; unused imports dropped.
- Docs: ADR-007 written; OQ-TLS-10 -> resolved-by-deviation;
  client.md/server.md/overview/README/task postscript synced
  (incl. the strict-foreign-server limit in ADR-007 §Limits).

Verification: cargo test 81 / --features tcp 94 / --all-features
105 green; clippy -D warnings clean (default + all-features); fmt
clean; cargo doc warning-free.

tasks/handshake-tests.md

@@ -82,6 +82,29 @@ The four handshake-level gaps (review 001 §U-3):
   stays inside it)
 - tasks/integration-suite.md (the existing suite this extends)
 
+## Postscript (2026-09-11 — the OQ-TLS-10 resolution)
+
+The open gap above is now **resolved by ADR-007** (deviation from
+alknet; OQ-TLS-10 → resolved). The two gap premises this task pinned
+are no longer true:
+
+- Suite 1's original premise (crate pin client ↔ raw-key server → ok)
+  is now **true**: `FingerprintPinVerifier::requires_raw_public_keys()`
+  derives from the pin format (`ed25519:` → `true`), so the crate's
+  own pin client completes against the crate's raw-key server —
+  `raw_key_server_path_completes_with_crate_pin_client` (no custom
+  verifier needed anymore).
+- Suite 3b's premise is now **inverted**: a raw-key client identity
+  presents its SPKI under the X.509 offer and
+  `AcceptAnyCertVerifier` accepts it end-to-end
+  (`raw_key_client_presents_spki_and_server_extracts_fingerprint`).
+- New pin: `ed25519_pin_against_x509_server_fails_closed_at_negotiation`
+  — a pin-format/cert-kind mismatch aborts at negotiation, never a
+  downgrade.
+
+Rationale and limits (incl. the strict-foreign-server caveat) in
+ADR-007; client.md/server.md notes flipped accordingly.
+
 ## Notes
 
 > **Major finding during implementation (recorded as OQ-TLS-10):** the