feat(#18): composite Bifrost endpoint — build_combined_app (Deliverable 1)

One ASGI app fronting BOTH the memory.* and affect.* planes (:8392), so a single
bound Worldtree session both remembers AND shows live PAD. Closes #18 end-to-end
(D2 PAD read-endpoint shipped v0.17.14; D1 was bifrost-blocked, now unparked by
bifrost 0.10.0's public build_combined_app + FR-1 resolved — zero Worldtree change).

- provider/combined.py: build_combined_provider_app wraps bifrost.consumer.build_combined_app
  over both stores + mounts the shared affect read route. Advertises both caps by store
  presence; per-route call-time isolation is bifrost's (INV-013).
- affect_store.py: extract add_affect_read_route shared helper (the D2 INV-007 promise —
  composite + standalone mount the SAME read route over the same affect.db, INV-011).
- opfeed.py: plane='combined' derives the OpEvent plane per request path
  (memory-call->memory, affect-call->affect, handshake->combined; INV-012).
- serve_combined.py + ratatoskr-combined-provider console script on :8392 (additive —
  standalone :8390/:8391 untouched, INV-014).
- contract: 18.contract.md § Deliverable 1 (INV-009..INV-014); D1 un-deferred.

Latent bug fixed (exposed by the contract-mandated memory `search` dispatch test running
through TestClient = a worker thread): open_memory_store lacked check_same_thread=False —
the SAME sqlite thread-safety bug already fixed in the affect store (D2). The composite
serves the memory plane over HTTP, so a memory-call on uvicorn's threadpool would trip it.
Fix: check_same_thread=False + PRAGMA busy_timeout=5000 (memory contract Concurrency note).

heid-code-review panel (Groa/Hulda/Regin): ZERO drift findings; the implementation matches
INV-009..INV-014 at function-block level. Folded the genuine test-fidelity fix (memory leg
describe_store -> search per the contract TEST) + added the PRE-001/PRE-002 guard tests.
Suite 486 -> 502 green.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "ratatoskr"
-version = "0.17.15"
+version = "0.17.16"
 description = "Worldtree Conversation API debug TUI — multi-pane observability dashboard"
 readme = "README.md"
 requires-python = ">=3.12"
@@ -50,6 +50,7 @@ ratatoskr                 = "ratatoskr.cli:main"
 ratatoskr-web             = "ratatoskr.web.entrypoint:main"
 ratatoskr-provider        = "ratatoskr.provider.serve:main"
 ratatoskr-memory-provider = "ratatoskr.provider.serve_memory:main"
+ratatoskr-combined-provider = "ratatoskr.provider.serve_combined:main"
 
 [project.urls]
 Repository = "https://gitea.phasefinal.com/vh/ratatoskr"