Introduce the platform adapter layer; extract window context behind it

Phase 3 groundwork of the improvement plan (PR 7 of the sequence). The Linux
work is a platform rewrite, so this first establishes the interface boundary
and moves an OS-specific piece behind it with Windows behavior preserved — no
new process.platform branches in shared code.

- app/platform/index.js is the single factory that selects a platform
  implementation; shared code asks it for adapters and never inspects
  process.platform itself.
- app/platform/interfaces.js documents the adapter contracts
  (WindowContextProvider, ClickSource, PowerPolicy) and the explicit click-
  source vocabulary.
- Extracted the foreground-window/element detection into per-OS adapters,
  verbatim from text-intel.js:
    app/platform/windows/window-context.js  (PowerShell UIAutomation)
    app/platform/linux/window-context.js     (xprop)
    app/platform/darwin/window-context.js     (AppleScript)
  text-intel.js now delegates to the injected provider and its three
  platform-branching methods (and the now-dead child_process import) are gone.
- app/platform/linux/diagnostics.js detects session type, portal/PipeWire,
  xinput, readable input devices, and the resulting click/screen-capture
  profile, returning actionable messages for the UI. Exposed via a new
  platform:capabilities IPC + preload method.

This is behavior-preserving: the Windows/macOS/Linux window-context code is
the same, just relocated behind the factory, and the capture pipeline is
untouched.

Tests: platform selection for every OS, provider validity + null-object for
unsupported OS, the shared service delegating to an injected provider, Linux
capability detection (x11/xinput, Wayland-without-PipeWire messaging, no-click
fallback, evdev), the capability facade, and a guard that text-intel no longer
branches on process.platform. 268 unit tests pass; startup smoke passes and
the click self-test is unchanged (stream source, markers 3/3, burst 8/8).

Co-Authored-By: Claude Fable 5 <[email protected]>

app/platform/darwin/window-context.js

@@ -0,0 +1,43 @@
+'use strict';
+
+const { execFileSync } = require('node:child_process');
+
+/**
+ * macOS WindowContextProvider using AppleScript / System Events. Extracted
+ * verbatim from text-intel.js. macOS is not a primary support target, but the
+ * adapter is kept so the shared code has no `process.platform` branch and the
+ * behavior is preserved where it exists. Never throws.
+ */
+function createDarwinWindowContextProvider() {
+  return {
+    async collect() {
+      const script = `
+      set appName to ""
+      set windowTitle to ""
+      tell application "System Events"
+        try
+          set frontApp to first application process whose frontmost is true
+          set appName to name of frontApp
+          try
+            set windowTitle to name of front window of frontApp
+          end try
+        end try
+      end tell
+      return appName & linefeed & windowTitle
+    `;
+      try {
+        const result = execFileSync('osascript', ['-e', script], {
+          encoding: 'utf8',
+          stdio: ['ignore', 'pipe', 'pipe'],
+          timeout: 1200,
+        }).trimEnd();
+        const [appName = '', windowTitle = ''] = result.split(/\r?\n/);
+        return { appName, windowTitle };
+      } catch {
+        return { appName: '', windowTitle: '' };
+      }
+    },
+  };
+}
+
+module.exports = { createDarwinWindowContextProvider };