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.

docs/architecture/README.md

@@ -33,6 +33,7 @@ unresolved.
 | [004](decisions/004-accessor-surface.md) | Accepted | Complete accessors: `for_tcp_tls()` adopted; server borrows, client consumes |
 | [005](decisions/005-config-types-move-into-alktls.md) | Accepted | Identity + credentials + fingerprint types move into alktls; auth layer stays out |
 | [006](decisions/006-module-layout-and-tests.md) | Accepted | Eight-module layout; seed tests + integration invariant pins |
+| [007](decisions/007-cert-type-negotiation.md) | Accepted | RFC 7250 cert-type negotiation: the offer follows the identity (deviation from alknet; resolves OQ-TLS-10) |
 
 ## Lifecycle