refactor(session): make ModelLane the provider boundary (#979) (#989)

* refactor(session): make ModelLane the provider boundary (#979)

## Summary

This closes the model-lane ownership gap left by #832: `ChatSession` no longer stores raw provider/client handles. `ResolvedModelBinding` now carries the provider, client, model, capabilities, registry generation, and backend-auth configuration as one coherent snapshot.

- Atomically rebind existing sessions after model-registry changes while pinning each in-flight send, fallback, judge, output guard, task agent, title, compaction, perception, and voice operation to its initiating principal and binding.
- Fence UI publication, canonical trajectory folds, durable writes, streams, retries, child scopes, and judge work by generation. Stop can hand off to a successor without accepting late state; cancelled tools retain typed effect receipts, and concurrent approval batches resolve by exact cycle or call.
- Make create, fork, open, close, and delete race-safe with hidden `creating` reservations, incarnation-aware state tails, and an ACL-rechecked transaction that clones checkpoint-bounded history, configuration, project/persona state, and attachment references.
- Extend REST/OpenAPI and Python/TypeScript SDK contracts for create/fork inputs, routed-create metadata, live-workstream probes, targeted approvals, and structured cancellation results.
- Update architecture, storage, authentication, judge, channel, console, API, and SDK documentation, including regenerated architecture diagrams and OpenAPI artifacts.

## Validation

- SQLite suite: 11,188 passed, 9 skipped, 10 deselected
- PostgreSQL suite: 11,195 passed, 2 skipped, 10 deselected
- Live backend: 3 passed
- SSE recovery: 6 passed; browser recovery harness passed all scenarios
- Ruff: clean; 595 files correctly formatted
- mypy: 243 source files clean
- TypeScript: typecheck/build and 35 tests passed
- OpenAPI artifacts fresh; all 14 changed diagrams reproduce byte-for-byte
- `git diff --check` and Git LFS integrity clean

Closes #979.

* fix(deps): update nanoid for GHSA-2v37-7h3g-55p8

Refresh the transitive lock entry admitted by PostCSS so the TypeScript security gate no longer resolves the vulnerable custom-generator implementation.

Validation:
- npm ci
- npm audit --audit-level=moderate: 0 vulnerabilities
- TypeScript typecheck and build
- TypeScript tests: 35 passed

* fix(test): assert canonical model registry URLs

Replace prefix checks with exact canonical base URL assertions so the tests do not model incomplete URL validation.

Validation: tests/test_model_registry.py (185 passed); Ruff check/format; mypy.

tests/test_rerank.py

@@ -8,6 +8,7 @@ from types import SimpleNamespace
 import httpx
 import pytest
 
+from turnstone.core.model_turn import resolve_capabilities
 from turnstone.core.rerank import (
     CohereJinaRerankClient,
     RerankHit,
@@ -592,7 +593,6 @@ class TestModelCapabilitiesRerankFields:
         import dataclasses
 
         from turnstone.core.providers._protocol import ModelCapabilities
-        from turnstone.core.session import ChatSession
 
         base = ModelCapabilities()
         provider = SimpleNamespace(get_capabilities=lambda model: base)
@@ -605,8 +605,7 @@ class TestModelCapabilitiesRerankFields:
             }
         )
         registry = SimpleNamespace(get_config=lambda alias: cfg)
-        stub = SimpleNamespace(_registry=registry)
-        caps = ChatSession._resolve_capabilities(stub, provider, "m", "rr")
+        caps = resolve_capabilities(provider, "m", "rr", registry)
         assert isinstance(caps, ModelCapabilities)
         assert caps.rerank_threshold == 0.5
         assert caps.rerank_scale == "logit (sigmoid-normalised)"