feat: initial CompileGraph integration into MasterServer (#376)

## Summary

Initial integration of `CompileGraph` (#375) into `MasterServer`,
enabling basic end-to-end C++20 module support: on-demand PCM building,
dependency-ordered compilation, cascade invalidation on save, and
diagnostic integration.

This is a **first-pass implementation** — the core pipeline works, but
there are known areas for follow-up:

- PCM files go to system temp dir instead of `.clice/cache/`; no disk
cleanup on invalidation
- `run_build_drain` scans imports itself rather than delegating fully to
CompileGraph
- No incremental/partial rebuild (full PCM rebuild on any change)
- Cycle detection is tested at unit level but integration-level coverage
is minimal

## Changes

### Module dependency compilation (`master_server.cpp`)

Before sending a file to the stateful worker, `run_build_drain` now:

1. Scans imports via `scan_precise()` to discover module dependencies
2. Compiles each dep through `compile_graph->compile()`, which
recursively builds transitive PCMs
3. Handles implementation units — `module M;` implicitly needs the
interface PCM
4. Passes all built PCMs to the stateful worker, excluding the file's
own PCM
5. Skips compile on dep failure and resets `build_running` /
`drain_scheduled`
6. Re-lookups iterators after `co_await` to avoid use-after-invalidation

### Cascade invalidation (`didSave` / `didClose`)

- `didSave`: calls `compile_graph->update()` to mark transitive
dependents dirty, removes stale PCM paths, schedules rebuilds for open
dirtied files
- `didClose`: cancels in-flight compilations for the closed file

### Other fixes in this PR

- Debounce timers switched to `shared_ptr` to prevent use-after-free
when `didClose` destroys the timer mid-wait
- `fill_compile_args` returns `bool`; callers handle empty CDB
gracefully
- Adapt all `PositionMapper` call sites to the new `optional` return API
from eventide

## Test plan

- [x] 25 C++ unit tests for CompileGraph (cycles, partial failure,
cancel, update, empty graph)
- [x] 24 C++ integration tests with real clang PCM compilation
- [x] 3 worker-level module tests (BuildPCM, PCM-dependent compile,
multi-module)
- [x] 26 Python LSP integration tests (single module through circular
deps, hover, error diagnostics)
- [x] 371 unit tests + 54 integration tests pass

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

---------

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

src/feature/code_completion.cpp

@@ -148,8 +148,8 @@ public:
 
         PositionMapper converter(content, encoding);
         auto replace_range = protocol::Range{
-            .start = converter.to_position(prefix.range.begin),
-            .end = converter.to_position(prefix.range.end),
+            .start = *converter.to_position(prefix.range.begin),
+            .end = *converter.to_position(prefix.range.end),
         };
 
         std::vector<protocol::CompletionItem> collected;