feat: implement remaining PRD features (10 beads)

Complete the PRD feature set with shell gating pipeline, cache
improvements, layout enhancements, and diagnostics:

- Shell: exec_gated with allowlist/denylist, circuit breaker, env merge
- Shell: parallel prefetch via std::thread::scope for cold renders
- Cache: TTL jitter (FNV-1a), config hash namespace, garbage collection
- Cache: diagnostic tracking (hit/miss, age) for dump-state
- Layout: gradual drop strategy (one-by-one vs tiered)
- Layout: render budget timer with graceful priority-based degradation
- Layout: breakpoint hysteresis to prevent preset toggling
- Width: detection source tracking for diagnostics
- CLI: --no-cache, --no-shell, --clear-cache, env var overrides
- Diagnostics: enhanced --dump-state with section timing and cache stats

Closes: bd-3oy, bd-62g, bd-khk, bd-3q1, bd-ywx, bd-3l2,
        bd-2vm, bd-1if, bd-2qr, bd-30o, bd-3ax, bd-3uw, bd-4b1

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

defaults.json

@@ -4,7 +4,7 @@
     "separator": " | ",
     "justify": "space-between",
     "vcs": "auto",
-    "cache_dir": "/tmp/claude-sl-{session_id}",
+    "cache_dir": "/tmp/claude-sl-{session_id}-{cache_version}-{config_hash}",
     "responsive": true,
     "breakpoints": {
       "narrow": 60,