fix(core): resolve root wikilinks by filename

[phernandez]

Why

Root-level WikiLinks written as filename stems never reached the existing .md fallback. Directly edited vaults also commonly use hyphens, underscores, or different case in links, leaving otherwise valid relations unresolved when a note has a descriptive title and permalink.

Closes #1253.

What changed

• allow the .md file-path retry for root-level identifiers, including uppercase extensions
• add a Unicode case-insensitive underscore/hyphen file-path alias fallback after all exact matches
• require forgiving aliases to identify exactly one file, so collisions remain unresolved
• apply the same backend-independent identity rules in both single-link and bulk resolution
• keep forgiving aliases out of create-or-update canonical identity checks
• cover root, relative, case, separator, Unicode, ambiguous-alias, and distinct-create behavior

Implementation notes

Exact permalink, title, and file-path identities retain precedence. The new alias lookup is deliberately last among strict link identity matches and does not guess when two canonical paths normalize to the same alias. Single resolution compares lightweight identity rows in Python because SQLite and Postgres do not share the same Unicode lower() behavior; bulk resolution builds the equivalent project index once.

Create-or-update writes use exact repository lookups only. This prevents a requested alpha-note.md create from moving or overwriting an existing alpha_note.md note before the alias collision exists.

Testing

• .venv/bin/python -m pytest -q --no-cov tests/services/test_upsert_entity_optimization.py tests/services/test_entity_service_disable_permalinks.py tests/services/test_link_resolver.py tests/services/test_bulk_link_resolver.py — 71 passed
• just fast-check
• just doctor

Risks and rollback

The link behavior is additive for previously unresolved links. Exact identifiers and canonical write identity are unchanged, and ambiguous aliases fail closed. Rollback is the single signed-off commit in this PR.

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