App capture actions and state machine

[Saber5656]

Source of truth: docs/issues/20-app-capture-actions.md in this repository (see PR #1 until merged). If this issue and the draft diverge, the draft wins.

Summary

Implement the app's four file-less capture actions — Capture Region, From Clipboard,
From Image File…, From PDF… — through the shared pipeline, completing the
CaptureCoordinator state machine with busy-rejection.

Context

Fills Issue 19's stubs using the Kit sources (Issues 12–15). Camera (Issue 21) and
delivery polish (Issue 23) are separate; in this issue delivery = clipboard write only
(the always-on baseline of DESIGN §9.5) so the flow is testable end-to-end.

Scope

• App/ScanMD/CaptureCoordinator.swift (real implementation), PipelineFactory.swift,
  OpenPanels.swift.

Detailed Requirements

1. CaptureCoordinator.request(_ kind: CaptureKind) (@MainActor):
   • if state != .idle → NSSound.beep(), drop the event (DESIGN §9.4), return.
   • transition per §9.4 driving a Task that runs acquire → pipeline → deliver with
     the heavy stages off the main actor; state updates back on main.
2. Source construction per kind:
   • .region → ScreenRegionSource (permission preflight inside the source; app is
     the TCC-responsible process here — remediation alert on .permissionDenied with an
     "Open System Settings" button using remediationURL from Issue 04).
   • .clipboard → ClipboardSource (with PDF delegation factory as in Issue 18).
   • .imageFile → NSOpenPanel: allowedContentTypes = [.png, .jpeg, .heic, .tiff, .gif, .webP], single selection, panel runs modally from the menu action; cancel →
     back to idle silently.
   • .pdf → NSOpenPanel with [.pdf]; after selection, if the document is large
     (> 20 pages) show a determinate progress in the menu bar icon tooltip (simple:
     state stays recognizing; per-page progress via a @Published pageProgress: (Int, Int)? — rendered as menu item text Recognizing page 3 of 12, disabled).
3. Config: coordinator reads ScanMDConfig via ConfigLoader at each capture (cheap,
   avoids staleness before Issue 22's file watching lands).
4. Delivery (baseline): ClipboardSink always; errors → NSAlert for now
   (notifications arrive in Issue 23; keep the error text = userMessage).
   .emptyResult shows an alert "No text found" (temporary UX, replaced in 23).
5. Concurrency: exactly one capture task; task stored, .cancelled results return to
   idle without alerts. App termination while busy cancels the task cleanly.
6. Unit tests (extracted, UI-free): reducer covers §9.4 rows incl. busy-drop; a fake
   source + fake sink drive a full coordinator run asserting state sequence
   idle→acquiring→recognizing→delivering→idle and clipboard-sink invocation; error
   and cancel paths return to idle with the right effect enum.
7. Manual checklist: region capture end-to-end from the menu (Markdown lands on
   clipboard — paste into a text editor) · clipboard capture of a screenshot · image
   file via panel · 12-page mixed PDF shows page progress and completes · double-invoke
   while busy beeps and drops · TCC-denied region shows the settings-link alert.

Acceptance Criteria

• All four actions work end-to-end on a dev build (manual checklist evidence).
• Reducer/state tests green; no capture path blocks the main thread (verify via
  Instruments hang detector once, note in PR).
• Busy-rejection behaves per §9.4 (beep + drop, no queueing).
• Permission alert deep-links to the Screen Recording pane.

Validation

Manual checklist + unit tests + a short screen recording (or step log) attached to the
PR.

Dependencies

Issues 12, 13, 14, 15, 19.

Non-goals

Camera (21), notifications/auto-save (23), settings/hotkey (22).

Design References

DESIGN §9.2, §9.4, §9.5 (baseline), §8.6 (remediation).