perf(hunt): the caller search prepares each product file once per change

call_sites rescanned every line of every product file for declarations on each call; with one name per
candidate row that was the whole cost of the source producer. code_index prepares a file once (the names each
line declares), call_sites takes it, and callers_of keeps a cache of prepared files the producer context owns.
Same calls, tested against the unprepared scan. One Laravel change of 33 rows: 249 seconds before this pair of
commits, 20 now.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

src/openultrasast/hunt/producers.py

@@ -56,6 +56,7 @@ class Context:
     spans: dict[str, tuple[source.FunctionRange, ...]] = field(default_factory=dict)
     range_errors: dict[str, str] = field(default_factory=dict)
     irs: dict[str, FileIR] = field(default_factory=dict)
+    call_index: dict[str, tuple[source.CodeLine, ...]] = field(default_factory=dict)
     diff_bytes: int = 0
 
     @cached_property
@@ -211,7 +212,9 @@ def _source(root: Path, change: Change, path: str, line: int, ctx: Context) -> d
     ctx.read(path)
     if path in ctx.range_errors:
         raise _Absent(ctx.range_errors[path])
-    facts = source.facts_for(root, path, line, entry_points=ctx.entry_points, files=ctx.product_files, texts=ctx.texts)
+    facts = source.facts_for(
+        root, path, line, entry_points=ctx.entry_points, files=ctx.product_files, texts=ctx.texts, index=ctx.call_index
+    )
     # Select the occurrence at this line when several methods share a name.
     spans = [r for r in ctx.ranges(path) if r.name == facts.function and r.start <= line <= r.end]
     span = min(spans, key=lambda r: r.end - r.start) if spans else None