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>

src/color.rs

@@ -49,6 +49,7 @@ pub fn resolve_color(name: &str, theme: Theme, palette: &ThemeColors) -> String
 }
 
 /// Determine whether color output should be used.
+/// Precedence: NO_COLOR > --color= CLI flag > CLAUDE_STATUSLINE_COLOR env > config
 pub fn should_use_color(cli_color: Option<&str>, config_color: &crate::config::ColorMode) -> bool {
     if std::env::var("NO_COLOR").is_ok() {
         return false;
@@ -62,6 +63,14 @@ pub fn should_use_color(cli_color: Option<&str>, config_color: &crate::config::C
         };
     }
 
+    if let Ok(env_color) = std::env::var("CLAUDE_STATUSLINE_COLOR") {
+        return match env_color.as_str() {
+            "always" => true,
+            "never" => false,
+            _ => atty_stdout(),
+        };
+    }
+
     match config_color {
         crate::config::ColorMode::Always => true,
         crate::config::ColorMode::Never => false,