perf(javascript): hash each semantic evidence context once (#1457)

The semantic graph's evidence-context registry canonicalized the full
provenance context three times for every node, relation, unknown and
fingerprint: once to hash its ID and twice more to compare against the
stored row. A file has only a few distinct contexts, so nearly all of
that work repeated a known answer.

Interning now keys contexts by their canonical text. Each fact
canonicalizes once, and only a new context is hashed and checked for a
digest collision. IDs and output are unchanged; Obsidian's 334 MB
Evidence is byte-identical and analysis drops from about 47.0 s to
44.4 s.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/domain/javascript/javascriptSemanticGraph.ts

@@ -56,6 +56,9 @@ const evidenceContextId = (context: ApplicationGraphEvidenceContext): string =>
 /** Graph-scoped owner for canonical provenance shared by semantic facts. */
 export class JavaScriptSemanticEvidenceContextRegistry {
   readonly #contexts = new Map<string, JavaScriptSemanticEvidenceContext>();
+  // Facts share a few contexts, so key them by canonical text and hash each
+  // distinct context once instead of on every fact.
+  readonly #idsByCanonicalContext = new Map<string, string>();
 
   /** Intern normalized non-location provenance at fact construction. */
   intern(
@@ -63,18 +66,21 @@ export class JavaScriptSemanticEvidenceContextRegistry {
   ): JavaScriptSemanticEvidenceReference {
     const normalized = normalizeEvidence(evidence);
     const { location, ...context } = normalized;
+    const canonicalContext = canonicalJson(
+      context,
+      "JavaScript semantic graph",
+    );
+    const internedId = this.#idsByCanonicalContext.get(canonicalContext);
+    if (internedId !== undefined) return { context_id: internedId, location };
     const contextId = evidenceContextId(context);
-    const row = { ...context, context_id: contextId };
-    const existing = this.#contexts.get(contextId);
-    if (
-      existing !== undefined &&
-      canonicalJson(existing, "JavaScript semantic graph") !==
-        canonicalJson(row, "JavaScript semantic graph")
-    )
+    // This canonical context is new, so a row already stored under its
+    // digest holds different provenance.
+    if (this.#contexts.has(contextId))
       throw new TypeError(
         "JavaScript semantic evidence context digest collision",
       );
-    this.#contexts.set(contextId, row);
+    this.#contexts.set(contextId, { ...context, context_id: contextId });
+    this.#idsByCanonicalContext.set(canonicalContext, contextId);
     return { context_id: contextId, location };
   }
 
@@ -101,6 +107,7 @@ export class JavaScriptSemanticEvidenceContextRegistry {
   /** Release interned contexts after the owning graph is finalized. */
   clear(): void {
     this.#contexts.clear();
+    this.#idsByCanonicalContext.clear();
   }
 }