refactor: public feature types and snapshot testing infrastructure (#442)

## Summary

- **Public feature types**: Move `SemanticToken`, `FoldingRange`,
`DocumentSymbol`, `InlayHint`, and `HintCategory` from internal `.cpp`
files to `feature.h` as public API types. Each feature now exposes two
overloads: a raw overload returning offset-based types and a protocol
overload that converts to LSP wire-format with explicit
`PositionEncoding`.
- **Snapshot testing**: Add corpus-driven snapshot tests using
`ASSERT_SNAPSHOT_GLOB` for semantic tokens, folding ranges, inlay hints,
document symbols, and TU index. Tests compile real C++ corpus files,
format output as YAML flow mappings, and diff against `.snap.yml`
baselines.
- **Test infrastructure**: Add `compile_file()` to `Tester`,
`yaml_str()` utility, `--corpus-dir` / `--snapshot-dir` CLI options, and
`--verbose` flag for unit tests. Migrate to kotatsu's unified
`kota::zest::Options` API.
- **Toolchain robustness**: Filter unknown cc1 args via
`clang::driver::getDriverOptTable()` to handle system compilers newer
than embedded LLVM.
- **Dependency bump**: Update kotatsu to 7381404 (unified zest Options,
out-param `from_json` API).

## Details

### Feature type changes
All five feature modules (`semantic_tokens`, `folding_ranges`,
`document_symbols`, `inlay_hints`, `document_links`) now follow the same
two-overload pattern. The raw overload returns offset-based structs
suitable for indexing and testing; the protocol overload adds
`PositionEncoding` conversion for LSP responses. `stateful_worker.cpp`
explicitly passes `PositionEncoding::UTF16` at every call site.

### Snapshot tests
Corpus files live in `tests/corpus/` (organized by language construct).
Snapshot baselines live in `tests/snapshots/<feature>/`. Format lambdas
are inlined directly in test bodies — no separate format functions for
single-use formatters. YAML output uses flow mappings (`- { key: value
}`) for compact, diffable baselines.

### cc1 arg filtering
`src/command/toolchain.cpp` now parses the cc1 argument list through
LLVM's driver option table and drops any args classified as
`UnknownClass`. This prevents compilation failures when the system
compiler emits flags that the embedded LLVM version doesn't recognize.

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

---------

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

src/server/workspace/config.cpp

@@ -156,13 +156,13 @@ std::optional<Config> Config::load(llvm::StringRef path, llvm::StringRef workspa
 }
 
 std::optional<Config> Config::load_from_json(llvm::StringRef json, llvm::StringRef workspace_root) {
-    auto result = kota::codec::json::from_json<Config>(json);
+    Config config{};
+    auto result = kota::codec::json::from_json(json, config);
     if(!result) {
-        LOG_WARN("Failed to parse initializationOptions JSON: {}", result.error().message());
+        LOG_WARN("Failed to parse initializationOptions JSON: {}", result.error().message);
         return std::nullopt;
     }
 
-    auto config = std::move(*result);
     config.apply_defaults(workspace_root);
     LOG_INFO("Loaded config from initializationOptions");
     return config;