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>
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