feat: complete Rust port of claude-statusline

Port the entire 2236-line bash statusline script to Rust.
Implements all 25 sections, 3-phase layout engine (render, priority
drop, flex/justify), file-based caching with flock, 9-level terminal
width detection, trend sparklines, and deep-merge JSON config.

Release binary: 864K with LTO. Render time: <1ms warm.

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

examples/dense.json

@@ -0,0 +1,10 @@
+{
+  "version": 1,
+  "layout": "dense",
+  "sections": {
+    "context_bar": {
+      "enabled": true, "priority": 1, "flex": true, "min_width": 10,
+      "bar_width": 8
+    }
+  }
+}