feat: header context protocol — queryContext, currentContext, switchContext (#398)

## Summary

Add three LSP protocol extensions that allow users to manage compilation
contexts for header files and source files with multiple CDB entries.

### Protocol extensions (`protocol.h`)

| Command | Purpose |
|---------|---------|
| `clice/queryContext` | List all possible contexts for a file. Headers
→ host source files; sources → CDB entries. Paginated (10 per page,
`offset` param). |
| `clice/currentContext` | Query the active context override for a file
(null if default). |
| `clice/switchContext` | Set the active context, invalidate caches,
trigger recompilation. |

### Header context resolution (`master_server.cpp`,
`dependency_graph.cpp`)

- `find_host_sources()`: BFS the reverse include graph to find source
files that transitively include a header
- `find_include_chain()`: BFS the forward include graph to find the
shortest include chain from host to header
- `resolve_header_context()`: walks the include chain, extracts content
before each `#include` directive, concatenates with `#line` markers into
a preamble file (hash-addressed under `.clice/header_context/`)
- `fill_header_context_args()`: uses the host source's CDB entry,
replaces source path with header path, injects `-include preamble.h`

### Compilation flow

- Default: headers compile as standalone files (no context)
- After `switchContext`: `fill_compile_args` checks `active_contexts`
first → uses host's CDB entry + preamble injection
- Fallback: if no CDB entry and no active context, auto-resolves via
`resolve_header_context`
- `#include` directive matching uses precise filename extraction from
`"..."` / `<...>`, not substring matching

### Source file multiple contexts (`multi_context` workspace)

- `queryContext` on a source file returns all CDB entries with
distinguishing labels (extracted from `-D`, `-O`, `-std=` flags)

### Test data

- `header_context/`: non-self-contained 3-level chain (`main.cpp` →
`utils.h` → `inner.h`), `types.h` provides `Point` struct
- `multi_context/`: single source with two CDB entries (`-DCONFIG_A`,
`-DCONFIG_B`)

### Tests (9 integration tests)

- queryContext returns host sources for headers
- queryContext returns CDB entries for source files
- currentContext defaults to null
- switchContext sets active context, currentContext reflects it
- Full flow: open → query → switch → hover works in non-self-contained
header
- Deep nested: switchContext + hover on `inner.h` (3 levels deep)
- Multiple CDB entries: queryContext returns both CONFIG_A and CONFIG_B

## Test plan
- [x] Unit tests: 465 passed
- [x] Integration tests: 113 passed (9 new header context tests)
- [x] Smoke test: 1/1 passed
- [ ] Manual VSCode testing

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

---------

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

tests/data/header_context/inner.h

@@ -0,0 +1,6 @@
+#pragma once
+
+// Non self-contained: uses Point from the include chain.
+inline Point inner_origin() {
+    return Point{0, 0};
+}

tests/data/header_context/main.cpp

@@ -0,0 +1,8 @@
+#include "types.h"
+#include "utils.h"
+
+int main() {
+    Point p{3, 4};
+    int d = calc(p);
+    return d;
+}

tests/data/header_context/types.h

@@ -0,0 +1,12 @@
+#pragma once
+
+struct Point {
+    int x;
+    int y;
+};
+
+inline int distance(Point a, Point b) {
+    int dx = a.x - b.x;
+    int dy = a.y - b.y;
+    return dx * dx + dy * dy;
+}

tests/data/header_context/utils.h

@@ -0,0 +1,9 @@
+// Non self-contained header: uses Point from types.h without including it.
+// Depends on the including source file to provide the types.h include.
+#pragma once
+
+#include "inner.h"
+
+inline int calc(Point p) {
+    return distance(p, inner_origin());
+}