feat(sdk): emitter affordance to record host-side projection failures

[MohammadHaroonAbuomar]

Closes #70.

What

When the HOST's own wire projection fails before a valid AgentContext exists for a point (e.g. a tool-call argument property getter throws during to-wire conversion at the chat seam), the host could only fail the run closed recordless — §11's reserved reasons cover interceptor/context failures the emitter can see, but a host-side projection fault happens before anything exists to emit.

This adds an emitter affordance that synthesizes and delivers the fail-closed record, in all five SDKs:

SDK	API
Rust	InterceptionEmitter::record_host_failure(point, HostFailure { detail, session_id, sequence, timestamp })
Python	emitter.record_host_failure(point, detail, *, session_id=, sequence=, timestamp=)
TypeScript	emitter.recordHostFailure(point, { detail, session_id, sequence, timestamp })
.NET	emitter.RecordHostFailure(point, detail, sessionId, sequence, timestamp)
Go	emitter.RecordHostFailure(point, HostFailure{Detail, SessionID, Sequence, Timestamp})

Record semantics

• The record is the existing §10.3 rejection shape: payload-free projection of a deny host_error:context_invalid, null identities under the declared provider, decided_by: null, no per-interceptor summaries, interceptors_registered as registered.
• No new reserved reason: host_error:context_invalid's cause already names "the host could not construct a schema-valid AgentContext"; spec/reserved-reasons.json is untouched (nothing for ACS's vendored copy to sync).
• Payload-free discipline: detail is a type name/path only, recorded as the verdict message and truncated by the same §10.3 projection as every combined verdict.
• Sequence integrity: the optional envelope facts carry the values the failed emission would have taken; the host SHOULD consume the next sequence number so records stay totally ordered (""/-1/absent when unknown). The record enters the same stream (sink, then buffer) as every emission.
• Enforcement modes: synthesis records in both modes; in evaluate_only the mode member keeps the record from implying a block happened (§8) — the action failed on its own, not on a verdict.
• All five bindings assemble the record through the core's finalize over a deliberately partial envelope basis (never §4-valid, so the rejection shape is forced), keeping cross-SDK behaviour pinned to one implementation. No record-shape or schema change; golden vectors unaffected.

Spec

• §10.3 gains a "Host projection failure" paragraph: MUST fail closed in enforce, SHOULD record; detail is type/path only (§14); sequence-integrity guidance; evaluate_only does not imply enforcement.
• §11's synthesis-site list adds §10.3.

Downstream

Unblocks microsoft/agent-framework#7564's recordless chat-seam projection-failure path and the same gap in the merged Python feature (adoption there is follow-up work; no agent-framework code is touched here).

Testing

Full local matrix mirroring CI: cargo fmt --check + clippy -D warnings + cargo test --locked --workspace --all-features (Rust); ruff format --check/ruff check + pytest 185 passed (Python); native build + zero binding drift + tsc + node --test 135 passed (TypeScript); dotnet format --verify-no-changes + -warnaserror build + 126 tests passed (.NET); gofmt/go vet/go test all packages ok (Go); scripts/check-version-consistency.py agrees at 0.1.0-alpha.5. Four new tests per SDK (rejection shape, unknown-envelope defaults, evaluate_only + sink delivery, projection truncation).