feat(index): piggyback indexing on PCH/PCM builds and open-file compiles (#402)

## Summary

Piggyback index construction onto existing compilation steps,
eliminating redundant recompilation in background indexing:

- **`TUIndex::build` gains `interested_only` parameter**: `true`
traverses only the main file's top-level decls; `false` (default)
traverses the full AST
- **PCH build indexes preamble headers**: stateless worker calls
`TUIndex::build(unit)` (full traversal) after successful `BuildPCH`,
clears `main_file_index`, serializes and sends back; master merges into
MergedIndex
- **PCM build indexes module interface**: stateless worker calls
`TUIndex::build(unit, true)` after successful `BuildPCM`; master merges
into MergedIndex
- **Open-file compile indexes main file**: stateful worker calls
`TUIndex::build(unit, true)` after successful `Compile`, serialized in
`CompileResult`
- **New `OpenFileIndex` in-memory structure**: master holds `FileIndex +
SymbolTable + buffer text` per open file — not persisted to disk, not
merged, discarded on close
- **Dual-source query path**: `query_index_relations`,
`lookup_symbol_at_position`, `find_symbol_definition_location`, all
hierarchy handlers, and `workspace/symbol` check `OpenFileIndex` first
(fresher), then fall back to `MergedIndex` (disk-indexed)
- **Background indexing skips open files**: checked via
`documents.count()`; on `didClose` the file is re-queued into
`index_queue`
- **`didSave` re-queues non-open dependents**: dirtied files from
`compile_graph->update()` that are not open get pushed into
`index_queue` for background re-indexing
- **Extract `lookup_occurrence` helper**: binary search + forward scan
picking the innermost (narrowest) match, replacing a broken
`while/break/break` pattern
- **Extract `find_symbol_info` helper**: consolidates 6 duplicated
"search open file indices then ProjectIndex" lookups into one method
- **`resolve_hierarchy_item` checks open file indices**: no longer
limited to ProjectIndex only

## Test plan

- [x] 465 unit tests pass
- [x] 105 integration tests pass (including all `test_index` cases:
GoToDefinition, FindReferences, CallHierarchy, TypeHierarchy,
WorkspaceSymbol)
- [x] Manual: open a file and immediately use GoToDefinition — should
work without waiting for background indexing
- [x] Manual: close a file and verify background indexing picks it up
and produces a MergedIndex shard

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 16bit-ykiko

src/index/tu_index.cpp

@@ -14,8 +14,8 @@ namespace {
 
 class Builder : public SemanticVisitor<Builder> {
 public:
-    Builder(TUIndex& result, CompilationUnitRef unit) :
-        SemanticVisitor<Builder>(unit, false), result(result) {
+    Builder(TUIndex& result, CompilationUnitRef unit, bool interested_only) :
+        SemanticVisitor<Builder>(unit, interested_only), result(result) {
         result.graph = IncludeGraph::from(unit);
     }
 
@@ -188,11 +188,11 @@ std::array<std::uint8_t, 32> FileIndex::hash() {
     return hasher.final();
 }
 
-TUIndex TUIndex::build(CompilationUnitRef unit) {
+TUIndex TUIndex::build(CompilationUnitRef unit, bool interested_only) {
     TUIndex index;
     index.built_at = unit.build_at();
 
-    Builder builder(index, unit);
+    Builder builder(index, unit, interested_only);
     builder.build();
 
     return index;

src/index/tu_index.h

@@ -85,7 +85,7 @@ struct TUIndex {
 
     FileIndex main_file_index;
 
-    static TUIndex build(CompilationUnitRef unit);
+    static TUIndex build(CompilationUnitRef unit, bool interested_only = false);
 
     void serialize(llvm::raw_ostream& os) const;