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/user-config.json

@@ -0,0 +1,23 @@
+{
+  "comment": "User config — only include overrides. Merged with defaults.json at runtime.",
+  "layout": "verbose",
+  "global": {
+    "justify": "left",
+    "width_margin": 4
+  },
+  "sections": {
+    "version": {
+      "enabled": true
+    },
+    "hostname": {
+      "enabled": false
+    },
+    "cost": {
+      "thresholds": {
+        "warn": 2.00,
+        "danger": 5.00,
+        "critical": 10.00
+      }
+    }
+  }
+}