Implement Advisory Canonicalization and Backfill Migration

- Added AdvisoryCanonicalizer for canonicalizing advisory identifiers.
- Created EnsureAdvisoryCanonicalKeyBackfillMigration to populate advisory_key and links in advisory_raw documents.
- Introduced FileSurfaceManifestStore for managing surface manifests with file system backing.
- Developed ISurfaceManifestReader and ISurfaceManifestWriter interfaces for reading and writing manifests.
- Implemented SurfaceManifestPathBuilder for constructing paths and URIs for surface manifests.
- Added tests for FileSurfaceManifestStore to ensure correct functionality and deterministic behavior.
- Updated documentation for new features and migration steps.

docs/dev/lnm-determinism-tests.md

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+# Link-Not-Merge Determinism Test Plan
+
+**Task:** MERGE-LNM-21-003 — replace legacy merge determinism suites with observation/linkset regressions now that `NoMergeEnabled` is defaulted to `true`.
+
+## Objectives
+- Validate raw advisory documents remain byte-stable through observation/linkset materialisation.
+- Ensure conflicts detected during linkset building surface in telemetry and persisted artifacts without merge-side mutation.
+- Keep canonical hash output stable for exports/evidence bundles after repeated runs.
+
+## Test Coverage Outline
+1. **Raw → Observation determinism**
+   - Feed canonical advisory raw fixtures containing mixed casing, duplicate aliases, and provenance metadata.
+   - Assert repeated runs of `AdvisoryObservationFactory` emit identical observations (structural equality + canonical JSON hash).
+   - Verify raw linkset payload retains original ordering/whitespace while canonical linkset stays normalised.
+   - Initial coverage implemented via `AdvisoryObservationFactoryTests.Create_IsDeterministicAcrossRuns` (core tests).
+
+2. **Linkset conflict surfacing**
+   - Build linksets from conflicting advisory observations (e.g., differing severity or status flags).
+   - Confirm conflict markers propagate to `AdvisoryLinkset` outputs and associated metrics/log records.
+   - Capture deterministic ordering of conflict explanations for evidence exports.
+
+3. **Evidence/export parity**
+   - Re-run observation/linkset pipelines against identical fixtures and assert resulting evidence manifests hash-identically.
+   - Track monotonic `supersedes` chains and ensure canonical link records include `PRIMARY` schemes.
+
+## Migration Steps
+- [ ] Retire `StellaOps.Concelier.Merge.Tests` determinism suites once observation/linkset equivalents land.
+- [ ] Introduce new regression fixtures under `StellaOps.Concelier.Core.Tests` (shared via `StellaOps.Concelier.Testing`).
+- [ ] Wire test helpers to Mongo in-memory harness for end-to-end parity runs.
+- [ ] Update documentation (`docs/migration/no-merge.md`) with validation checklist once new tests are green.
+
+_Pending_: execute suites on a workstation with the .NET 10 preview SDK; local environment lacks a functioning CLI, so validation runs must happen downstream.