feat: implement index system with LSP query handlers (#382)

## Summary

Implement the complete index system for cross-file LSP features. This
adds persistent two-tier indexing (ProjectIndex + per-file MergedIndex
shards), background indexing triggered on idle, and index-based query
handlers for major LSP requests.

### Index Data Layer (`src/index/`)

- **TUIndex**: Add binary serialization/deserialization via FlatBuffers,
enabling IPC between stateless worker and master server
- **ProjectIndex**: Add symbol name/kind storage, `PathPool` path
normalization (backslash -> forward slash), and binary persistence
- **MergedIndex**: Add `content` field to store file content for
reliable offset<->position mapping; add `removed` bitmap for garbage
collection of deleted entries; filter removed IDs in `lookup()` queries
- **schema.fbs**: Add TUIndex tables, `Symbol.name` field,
`MergedIndex.removed` bitmap and `MergedIndex.content` string

### Server (`src/server/`)

- **Background indexing**: Idle-triggered coroutine dequeues files from
CDB, dispatches `IndexParams` to stateless workers, merges returned
`TUIndex` into ProjectIndex/MergedIndex, and persists to `.clice/index/`
- **Index persistence**: `save_index()` / `load_index()` for startup
restoration; only rewrites shards flagged `need_rewrite()`
- **LSP handlers**:
- `textDocument/definition` -- index-first lookup with stateful worker
fallback
  - `textDocument/references` -- cross-file reference query via index
- `callHierarchy/prepare`, `incomingCalls`, `outgoingCalls` --
Caller/Callee relation traversal
- `typeHierarchy/prepare`, `supertypes`, `subtypes` -- Base/Derived
relation traversal
- `workspace/symbol` -- case-insensitive substring search over
ProjectIndex symbols
- **Stateless worker**: Add `Index` request handler that builds
`TUIndex` from compiled AST and returns serialized data
- **Config**: Add `enable_indexing` (default true) and `idle_timeout_ms`
(default 3000ms)

### Fixes and Cross-platform

- **ElaboratedType handling** in `decl_of()` for correct Base/Derived
relation emission
- **Windows path normalization** in `PathPool::intern()` and
`ProjectIndex::from()` (backslash -> forward slash)
- **`.gitattributes`**: Force LF in `tests/data/**` to prevent CRLF
byte-offset mismatches on Windows CI
- **Test fixture**: Clean `.clice/` before each test for hermetic index
state

### Tests

- **370-line** `index_query_tests.cpp`: unit tests for occurrence
lookup, relation queries, content retrieval, removed bitmap filtering
- **282-line** `test_index.py`: E2E integration tests for
GoToDefinition, FindReferences, CallHierarchy
(prepare/incoming/outgoing), TypeHierarchy
(prepare/supertypes/subtypes), WorkspaceSymbol
- Updated existing MergedIndex and ProjectIndex tests for new schema
fields

## Test plan

- [x] 414 C++ unit tests pass (including new IndexQuery, MergedIndex,
ProjectIndex tests)
- [x] 69 Python integration tests pass (including 10 new index feature
tests)
- [x] CI green on Linux, macOS, Windows
- [ ] Manual smoke test with VSCode extension

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

tests/unit/index/project_index_tests.cpp

@@ -163,6 +163,43 @@ TEST_CASE(FileIdsMapCorrectness) {
     }
 }
 
+TEST_CASE(NameSurvivesRoundTrip) {
+    index::TUIndex tu;
+    ASSERT_TRUE(build_and_index(R"(
+            int my_variable = 42;
+            void my_function() {}
+        )",
+                                tu));
+
+    index::ProjectIndex project;
+    project.merge(tu);
+
+    // Verify names are populated after merge.
+    bool found_var = false;
+    bool found_func = false;
+    for(auto& [hash, symbol]: project.symbols) {
+        if(symbol.name == "my_variable")
+            found_var = true;
+        if(symbol.name == "my_function")
+            found_func = true;
+    }
+    ASSERT_TRUE(found_var);
+    ASSERT_TRUE(found_func);
+
+    // Serialize and deserialize.
+    llvm::SmallString<4096> buf;
+    llvm::raw_svector_ostream os(buf);
+    project.serialize(os);
+    auto restored = index::ProjectIndex::from(buf.data());
+
+    // Verify names survive round-trip.
+    for(auto& [hash, symbol]: project.symbols) {
+        ASSERT_TRUE(restored.symbols.contains(hash));
+        ASSERT_EQ(restored.symbols[hash].name, symbol.name);
+        ASSERT_EQ(restored.symbols[hash].kind.value(), symbol.kind.value());
+    }
+}
+
 };  // TEST_SUITE(ProjectIndex)
 }  // namespace
 }  // namespace clice::testing