fix: width detection prioritizes dynamic sources over config

Config `width` was position #3 in the detection chain, overriding all
dynamic detection (ioctl, process tree, stty, etc). This meant the
statusline couldn't adapt to terminal/pane resizes.

Now config `width` serves two roles:
- Max cap on dynamically detected widths (prevents absurd widths)
- Fallback when all dynamic detection methods fail

Also adds:
- ioctl on stderr (works when stdout is piped)
- stdin JSON `terminal_width` field for Claude Code to pass width
- Distinct diagnostic sources: config_cap, config_fallback, stdin_json

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

statusline.json

@@ -0,0 +1,28 @@
+{
+  "$schema": "./schema.json",
+  "global": {
+    "justify": "spread",
+    "theme": "dark",
+    "responsive": true,
+    "width": 500,
+    "width_margin": 20
+  },
+  "glyphs": {
+    "enabled": true
+  },
+  "layout": "verbose",
+  "sections": {
+    "context_usage": {
+      "enabled": true
+    },
+    "context_trend": {
+      "enabled": true
+    },
+    "cost_trend": {
+      "enabled": true
+    },
+    "load": {
+      "enabled": false
+    }
+  }
+}