fix(db): dedup edges with a UNIQUE identity index so INSERT OR IGNORE works (#1034) (#1050)

`insertEdge` has always used `INSERT OR IGNORE`, but the edges table carried
no UNIQUE constraint — only an autoincrement PK and non-unique indexes — so
`OR IGNORE` had nothing to conflict on and behaved like a plain INSERT.
Whenever two extraction/resolution passes emitted the same edge (e.g. a
return type captured by both a type-reference and a value-reference pass),
the graph stored byte-identical duplicate rows: ~527 on this repo, inflating
edge counts and letting callers/impact list the same relationship twice.

Add a UNIQUE identity index on (source, target, kind, IFNULL(line,-1),
IFNULL(col,-1)) — in schema.sql for fresh databases and migration v6 (dedup
existing rows, then create the index) for existing ones. IFNULL folds the
nullable line/col so coordinate-less edges (synthesized / file-level) dedup
too; SQLite otherwise treats each NULL as distinct. Distinct call sites
(same source/target/kind, different line/col) are preserved — only
byte-identical structural duplicates collapse. This is the storage-layer
invariant the reporter identified: it makes OR IGNORE keep its promise and
catches every double-emit, present and future, rather than chasing each
emitting pass.

Migration v6 is deterministic (keeps the lowest id per identity group) and
idempotent (IF NOT EXISTS index; no-op DELETE once unique). The DELETE's
GROUP BY matches the index expression exactly so creation can't fail on a
leftover pair.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -16,6 +16,7 @@ and adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 - On Windows, CodeGraph's background server now shuts down cleanly instead of occasionally aborting with a crash error. When the indexed project contained a nested repository (a submodule or embedded clone), stopping the server could race the file watcher's teardown and exit with a Windows crash code rather than a clean exit. Shutdown now lets that teardown finish first, so the server stops cleanly and promptly. (Windows only; other platforms were unaffected.) (#1033)
 - C++ classes that inherit from a templated base — `class Widget : public Base<int>`, a CRTP base like `class App : public CRTPBase<App>`, or a struct inheriting a template — are now linked to that base class in the graph. Previously the template arguments (`<int>`) made the inheritance go unrecognized, so these classes looked like they inherited from nothing and impact/callers analysis stopped at the boundary; the connection is now followed like any other base class. Thanks @ryancu7 for the report. (#1043)
 - C++ objects constructed on the stack — `Calculator calc(0)` or `Widget w{1, 2}` — now record that the enclosing function instantiates that class, the same as heap construction (`new Calculator(0)`) already did. Previously only the `new` form was tracked, so a function that built objects with the ordinary stack syntax looked like it didn't construct them and the dependency was missing from impact/callers. Thanks @Dshuishui for the report. (#1035)
+- The graph no longer stores duplicate copies of the same relationship. The same dependency between the same two symbols at the same spot could be recorded more than once, which inflated edge counts and let callers/impact results list a relationship twice. Each relationship is now stored exactly once, and existing projects are de-duplicated automatically the next time CodeGraph opens them. Thanks @inth3shadows for the detailed report. (#1034)
 
 
 ## [1.1.2] - 2026-06-28