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/index.js

@@ -0,0 +1,66 @@
+'use strict';
+
+/**
+ * The single factory that selects a platform implementation. The rest of the
+ * app depends on the interfaces in ./interfaces.js and asks this module for a
+ * concrete adapter — it never branches on `process.platform` itself.
+ *
+ * As Linux runtime capture is implemented, its ClickSource / ScreenFrameSource
+ * adapters are added here; today this provides the WindowContextProvider for
+ * every platform and the Linux capability diagnostics.
+ */
+
+const { assertWindowContextProvider } = require('./interfaces');
+
+function detectPlatform(platform = process.platform) {
+  if (platform === 'win32') return 'windows';
+  if (platform === 'darwin') return 'darwin';
+  if (platform === 'linux') return 'linux';
+  return 'unsupported';
+}
+
+/**
+ * Build the WindowContextProvider for the current OS. `platform` is injectable
+ * so the selection logic is unit-testable off the target OS.
+ */
+function createWindowContextProvider({ platform = process.platform } = {}) {
+  const os = detectPlatform(platform);
+  let provider;
+  switch (os) {
+    case 'windows':
+      provider = require('./windows/window-context').createWindowsWindowContextProvider();
+      break;
+    case 'darwin':
+      provider = require('./darwin/window-context').createDarwinWindowContextProvider();
+      break;
+    case 'linux':
+      provider = require('./linux/window-context').createLinuxWindowContextProvider();
+      break;
+    default:
+      // Unsupported OS: a null-object provider so callers still work.
+      provider = { async collect() { return { appName: '', windowTitle: '' }; } };
+  }
+  return assertWindowContextProvider(provider);
+}
+
+/**
+ * Capability profile for the current OS (used by diagnostics UI). Only Linux
+ * has a rich profile today; other platforms report their OS and a capable
+ * baseline.
+ */
+function detectCapabilities({ platform = process.platform, env = process.env } = {}) {
+  const os = detectPlatform(platform);
+  if (os === 'linux') {
+    return require('./linux/diagnostics').detectLinuxCapabilities({ env });
+  }
+  return {
+    os,
+    sessionType: os,
+    isWayland: false,
+    clickCapture: os === 'windows' ? 'windows-hook' : os,
+    screenCapture: os,
+    messages: [],
+  };
+}
+
+module.exports = { detectPlatform, createWindowContextProvider, detectCapabilities };