refactor(sources): RFC 002 §9 scaffolding — BaseSourceAdapter, registry, PalaceContext

Lands the read-side contract so third-party adapter authors (@Perseusxrltd,
@JakobSachs, @adv3nt3, @zendesk-thittesdorf, @mfhens, @roip, @MrDys) have a
stable target matching what RFC 001 §10 landed on the write side in #995.

Scope (this PR):

- mempalace/sources/base.py: BaseSourceAdapter ABC with kwargs-only
  ingest() / describe_schema() and default is_current() / source_summary()
  / close() (§1.1–1.2). Typed records: SourceRef, SourceItemMetadata,
  DrawerRecord, RouteHint, SourceSummary, AdapterSchema, FieldSpec (§1.3,
  §5.2). Error classes: SourceNotFoundError, AuthRequiredError,
  AdapterClosedError, TransformationViolationError, SchemaConformanceError
  (§2.7). Class-level identity contract: name / adapter_version /
  capabilities / supported_modes / declared_transformations /
  default_privacy_class (§2.1, §1.4, §1.5, §6).

- mempalace/sources/transforms.py: reference implementations of the 13
  reserved transformations (§1.4) — utf8_replace_invalid, newline_normalize,
  whitespace_trim, whitespace_collapse_internal, line_trim, line_join_spaces,
  blank_line_drop — as pure functions, plus identity shims for the six
  adapter-specific ones (strip_tool_chrome, tool_result_truncate,
  tool_result_omitted, spellcheck_user, synthesized_marker,
  speaker_role_assignment) that the conversations adapter will override
  when migrated. get_transformation(name) resolves by reserved name.

- mempalace/sources/registry.py: entry-point discovery via
  importlib.metadata.entry_points(group="mempalace.sources") + explicit
  register()/unregister() surface (§3.1–3.2). resolve_adapter_for_source()
  implements the §3.3 priority order; crucially, no auto-detection on the
  read side (§3.3 is explicit about that — user intent never inferred from
  on-disk artifacts).

- mempalace/sources/context.py: PalaceContext facade (§9) bundling the
  drawer/closet collections, knowledge graph, palace path, adapter identity,
  and progress hooks core passes into adapter.ingest(). upsert_drawer()
  applies the spec-mandated adapter_name/adapter_version stamps from §5.1.
  skip_current_item() signals laziness; emit() dispatches to hooks and
  swallows hook exceptions.

- mempalace/knowledge_graph.py: add_triple() gains optional source_drawer_id
  and adapter_name kwargs (§5.5). Backwards-compatible column migration
  auto-adds the new columns on open of a pre-RFC 002 palace (PRAGMA
  table_info then ALTER TABLE ADD COLUMN), matching the pattern used for
  any new palace-side provenance fields.

- pyproject.toml: mempalace.sources entry-point group declared. Empty on
  the first-party side for now — miners migrate in a follow-up; the group
  being present means third-party packages can begin registering today.

Out of scope (explicit follow-ups):

- miner.py → mempalace/sources/filesystem.py. Behavior-preserving rename
  that also moves READABLE_EXTENSIONS, detect_room(), detect_hall() into
  the adapter (§9). Larger refactor; lands separately.
- convo_miner.py + normalize.py → mempalace/sources/conversations.py. The
  format-detection if-chain in normalize.py becomes per-format plugins;
  declared_transformations enumerates what the current pipeline already
  does to source bytes (§1.4 existing-code mapping).
- Closet post-step wired into the conversations adapter (§1.7).
- CLI --source flag + --mode deprecation alias (§3.3).
- MCP mempalace_mine tool source parameter.
- AbstractSourceAdapterContractSuite (§7.1–7.3): byte-preservation round-
  trip and declared-transformation round-trip tests.
- Privacy-class floor enforcement (§6.2); depends on #389 for
  secrets_possible scanning.

Tests: 1018 passed (up from ~990 on develop), +27 targeted tests covering
the ABC instantiation rules, typed records, all reserved transformations,
the registry register/get/unregister surface, PalaceContext upsert + skip +
emit semantics, and both the new KG provenance kwargs and backwards-
compatible legacy-schema migration.

Refs: #989 (RFC 002 tracking), #990 (RFC 002 spec), #995 (RFC 001 §10
cleanup — sibling PR on the write side).

pyproject.toml

@@ -40,6 +40,12 @@ mempalace = "mempalace.cli:main"
 [project.entry-points."mempalace.backends"]
 chroma = "mempalace.backends.chroma:ChromaBackend"
 
+# RFC 002 source-adapter entry-point group. Core publishes no first-party
+# adapters under this group yet; ``miner.py`` and ``convo_miner.py`` migrate
+# onto ``BaseSourceAdapter`` in a follow-up PR. Third-party adapter packages
+# (``mempalace-source-cursor``, ``mempalace-source-git``, …) register here.
+[project.entry-points."mempalace.sources"]
+
 [project.optional-dependencies]
 dev = ["pytest>=7.0", "pytest-cov>=4.0", "ruff>=0.4.0", "psutil>=5.9"]
 spellcheck = ["autocorrect>=2.0"]