tree_sitter test coverage + DI seam, ratchet floor to 95

Add `colorForRole` switch-arm tests (every role → token mapping plus
the unknown-role fallback). Introduce a DI seam in `TreeSitterService`
and `TreeSitterLib` so tests can substitute the FFI surface and asset
loaders without dlopen'ing `libtree-sitter.so` —
`TreeSitterLib.testing(...)` takes named per-function overrides with
safe no-op defaults, and `TreeSitterLib.fromDynamicLibrary(...)` lets
the smoke test load the vendored library explicitly. Production
paths (`TreeSitterService.shared`, `TreeSitterLib.instance`) are
unchanged.

Fake-FFI tests walk every branch of `_init`, `_loadGrammar`,
`highlight`, and `dispose`. The smoke test catches FFI-signature
regressions the fakes can't, by exercising the real native library
end-to-end on Linux. Together this takes `tree_sitter_service.dart`
from 17% to 96% and crosses the global 95% target — closing out the
D-66 line-coverage epic.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pubspec.yaml

@@ -18,7 +18,7 @@ repository: https://github.com/postmeridiem/clide
 
 # Pre-push line-coverage floor. Ratchets up only — see D-66.
 # Reading: `awk -F: '/^coverage_floor:/ {gsub(/ /,"",$2); print $2}' pubspec.yaml`.
-coverage_floor: 93
+coverage_floor: 95
 
 # Project metadata (was project.yaml, folded in per D-056).
 # version: above is the single source of truth. The Makefile reads