spike(kernel): Stage-0 perf instrumentation (per-phase split + GPU timer)

Closes out Phase 0 with the measurement that explains the ~30fps: a per-frame
budget breakdown and a non-blocking GPU timer query.

- GlBridge: load EXT_disjoint_timer_query (glGenQueriesEXT/Begin/End/
  GetQueryObject*), gated by the GL extension string; report gpu_timer in the
  bring-up line. Absent -> graceful 'n/a'.
- PresentTarget::render(ctx, RenderTimings*): time clear/update/render-submit/
  present (CPU wall-clock) and bracket ctx->Render() with a 2-deep timer-query
  ring read back a frame late, so the REAL GPU fill time is measured without a
  glFinish stall. delete_queries in teardown.
- composite_frame: time the import+layout loop; accumulate all phases; emit a
  [perf-split] line (per-rendered-frame averages) alongside [perf].

Result (headless 1080p, Haswell+crocus == CF-AX3 GPU class): CPU work ~2ms
(import 0.7 / update 0.7 / render-submit 0.8), GPU fill (ctx->Render) ~10-15ms,
present dominated by the blocking fence wait. -> fill-bound; the whole-output
composite is the wall. Damage limiting is the lever, to be built properly in
the Phase 1 compositor (not hacked into this throwaway). --verify ALL PASS,
kernel suite green.

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