fix(mcp): resolve module-qualified symbol lookups (#173)

[colbymchenry]

Summary

• Treat :: as a token separator in the FTS5 query builder so stage_apply::run no longer collapses to the unsearchable stage_applyrun
• Extend matchesSymbol to accept ::, ., and / qualifier syntaxes, fall back to file-path containment when qualifiedName doesn't carry the module hierarchy (Rust / Python), and strip Rust path prefixes (crate, super, self)
• Stop falling through to fuzzy matches when a qualified lookup has no exact match — stage_apply::nonexistent_fn returns null instead of resolving to an unrelated rollback
• 9 new tests in __tests__/symbol-lookup.test.ts covering ::, multi-level, crate::-prefixed, /-style, bare-name, and the unknown-qualifier case

Why

#173 — codegraph_callees stage_apply::run returned "not found" against a repo with 7-9 sibling Rust modules each exporting pub async fn run. The bare-name run lookup was reported to silently resolve to an unrelated run_due_tasks (already addressed by findAllSymbols aggregation in 0.7.x — kept and pinned by test).

Root causes:

Layer	Problem	Fix
FTS5 query builder (src/db/queries.ts)	Strips : as a special char without splitting on ::, collapsing stage_apply::run to stage_applyrun	Replace :: with whitespace before the strip pass
matchesSymbol (src/mcp/tools.ts)	Only understood Parent.child (dot) qualifiers and required the qualifier to appear in qualifiedName	Accept ::, ., /. Fall back to file-path containment so Rust file-level functions (stage_apply.rs → stage_apply module) match. Strip Rust path prefixes (crate, super, self)
findSymbol (src/mcp/tools.ts)	Fell through to fuzzy text match on a qualified miss	Return null for qualified lookups with no qualifier-respecting match

End-to-end verification

Indexed https://github.com/kernex-dev/kernex-agent at the exact commit 7eb5d13 from the issue (59 files, 1,340 nodes):

findSymbol "stage_apply::run"        -> src/configurator/stage_apply.rs:43  (correct)
findSymbol "stage_detect::run"       -> src/configurator/stage_detect.rs:14 (correct)
findSymbol "stage_verify::run"       -> src/configurator/stage_verify.rs:36 (correct)
findSymbol "stage_apply::nonexistent_fn"  -> NOT FOUND (no fuzzy fallback)
findAllSymbols "run"                 -> 9 matches, with disambiguation note
findSymbol "render_and_write"        -> src/configurator/stage_apply.rs:177 (correct)

Test plan

• npx vitest run __tests__/symbol-lookup.test.ts — 9 new tests
• npm test — all 599 tests pass (zero regressions in pr19-improvements disambiguation tests, search-query-parser, mcp-initialize)
• npm run build — clean tsc
• End-to-end on the kernex-agent repo at the issue's reference commit (above)

Closes #173

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