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/custom-commands.json

@@ -0,0 +1,31 @@
+{
+  "version": 1,
+
+  "layout": [
+    ["model", "provider", "project", "vcs"],
+    ["context_bar", "cost", "duration", "ollama", "docker"]
+  ],
+
+  "custom": [
+    {
+      "id": "ollama",
+      "label": "ollama",
+      "command": "pgrep -x ollama >/dev/null && echo 'up' || echo 'down'",
+      "ttl": 30,
+      "priority": 3,
+      "color": {
+        "match": { "up": "green", "down": "dim" }
+      }
+    },
+    {
+      "id": "docker",
+      "label": "docker",
+      "command": "docker info >/dev/null 2>&1 && echo 'up' || echo 'down'",
+      "ttl": 60,
+      "priority": 3,
+      "color": {
+        "match": { "up": "green", "down": "dim" }
+      }
+    }
+  ]
+}