feat(formatting): wire up textDocument/formatting and rangeFormatting (#441)

## Summary

- Wire up the existing `document_format` feature to LSP via stateless
workers
- Add `Format` kind to stateless worker dispatch, with a lightweight
`forward_format` path in `Compiler` (no compilation/deps needed — just
file path + content)
- Register `textDocument/formatting` and `textDocument/rangeFormatting`
handlers with `scoped_pause`
- Style lookup uses `clang::format::getStyle` which walks parent
directories for `.clang-format`, matching clangd's behavior

## Test plan

- [x] 4 unit tests: simple format, range format, idempotent (no edits),
include sort
- [x] 3 integration tests: full document format (verifies applied edits
match expected output), range format, already-formatted no-op
- [x] Capability assertions added to `test_capabilities`
- [x] All existing tests pass (554 unit, 170 integration, 2 smoke)

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

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

## Release Notes

* **New Features**
* Added document formatting and range formatting capabilities to the LSP
server
* Formatting can target the entire document or a specific range of code
  * Server now advertises formatting support to LSP clients

* **Tests**
  * Added comprehensive test coverage for formatting functionality

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

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

tests/integration/features/test_server.py

@@ -34,6 +34,8 @@ async def test_capabilities(client, workspace):
     assert capability_enabled(caps.folding_range_provider)
     assert capability_enabled(caps.inlay_hint_provider)
     assert capability_enabled(caps.code_action_provider)
+    assert caps.document_formatting_provider is True
+    assert caps.document_range_formatting_provider is True
     assert caps.semantic_tokens_provider is not None