feat(frost/signing): enforce AttemptContextHash on receive (RFC-21 Phase-6 milestone)

[mswilkison]

Summary

Closes the Phase-6 milestone the RFC named but the implementation
skipped: receive callbacks now reject messages whose
`AttemptContextHash` does not match the session's bound
`AttemptContext`. Default builds and sessions without a
ROAST-attempt binding skip enforcement entirely, so the change
is observationally identical to pre-Phase-6 behaviour outside
the ROAST path.

Stacked on #3988 (M4 closure).

Why this matters

The Phase 1B `AttemptContextHash` field was structural-only
(present, 32 bytes) until now. Senders could populate it but
receivers ignored the value -- meaning a peer could send a
message bound to attempt N to a receiver running attempt N+1 of
the same session and the receiver would accept it as long as
`SessionID` matched. This PR closes that gap.

What lands

Surface	Behaviour
`verifyMessageAttemptContextHash(msg, sessionID)`	No binding → pass (legacy/default). Binding + matching hash → pass. Binding + missing hash → `ErrAttemptContextHashMissing`. Binding + mismatch → `ErrAttemptContextHashMismatch`.
`attemptContextHashCarrier` interface	One implementation covers all three FROST/tbtc-signer message types via their existing `GetAttemptContextHash` methods.
3 receive callbacks updated	After `shouldAcceptNativeFROSTMessage`, call `verifyMessageAttemptContextHash`. Failure → `evidence.RecordReject(senderID, "attempt_context_hash_mismatch")` so the policy can permanently exclude peers that consistently send stale-attempt messages.

Test coverage

File	Build	Cases
`attempt_context_binding_validation_frost_native_test.go`	`frost_native && frost_roast_retry`	5 (no-binding, matching, missing, mismatch, real-message integration with rebind)
`attempt_context_binding_validation_default_build_test.go`	`frost_native && !frost_roast_retry`	1 (default build always passes; rollback promise upheld)

Migration path

1. Phase 1B (already shipped): field structurally validated when present, optional otherwise.
2. This PR: enforced only when the session has a ROAST-attempt binding. Default builds and non-ROAST tagged sessions continue to ignore the field.
3. Future PR: once production has rolled out a version that populates the field on every outbound message, enforcement can be made unconditional.

Verification

Command	Result
`go build ./...` + tagged	both clean
`go test ./pkg/frost/...`	pass
`go test -tags 'frost_native frost_tbtc_signer'`	pass
`go test -tags 'frost_native frost_tbtc_signer frost_roast_retry'`	pass
`staticcheck -checks '-SA1019' ./pkg/frost/...`	silent
`go vet ./pkg/frost/...`	clean
`gofmt -l ./pkg/frost/signing/`	silent

Test plan

• CI green.
• Reviewer confirms the "no binding = skip enforcement" gate is acceptable (alternative: always enforce when build tag set, regardless of binding -- riskier during transitions).
• Reviewer confirms the failure-mode rejects record evidence rather than just dropping (so misbehaving peers accumulate exclusion-worthy counts).