docs: App Store screenshots placeholders + APPSTORE_METADATA.md

App Store Connect requires screenshots at 1290x2796 (6.7") and
1179x2556 (6.1") for the iPhone 17 Pro Max / 17 / Pro lineup,
with at least 3 and at most 10 images per size. The repository
previously had no screenshots in docs/.

This commit adds:
  - scripts/generate_screenshot_placeholders.py - PIL-based
    generator that produces 5x 6.7" + 5x 6.1" placeholders with
    status bar, mock device frame, keyboard mock, and headline /
    subtitle text.
  - docs/screenshots/6.7/ and 6.1/ with 5 placeholder PNGs each
    (10 total, all at the correct Apple-mandated dimensions).
  - docs/screenshots/README.md explaining the dimensions,
    generation script, and how to capture real Simulator
    screenshots via `xcrun simctl io booted screenshot`.
  - docs/APPSTORE_METADATA.md - a single source of truth for
    every field in App Store Connect: name, subtitle, URLs,
    pricing, description (<=4000 chars), promotional text
    (<=170 chars), keywords (<=100 chars), release notes
    (<=4000 chars), what's new, App Privacy answers
    (Data Not Collected), encryption declaration, and reviewer
    notes.
  - AUDIT_APPSTORE.md - the v0.1.2 pre-launch audit report
    with 6 P0 items, 5 P1 items, 4 P2 items, and a 9-item
    "cross-cutting observations" section addressing the items
    in the original brief that turned out to be non-issues
    (CFBundleURLTypes in keyboard ext, NSSupportsLiveText,
    AppGroup fatalError inconsistency, AudioCaptureService
    interaction with AVAudioSession, ASRService SFSpeechRecognizer
    references). Includes an Appendix A with the local
    xcodebuild build + test results.

Important: the placeholder PNGs are intentionally bland - they
exist so App Store Connect accepts the dimensions. They MUST
be replaced with real Simulator screenshots before the actual
upload, see docs/screenshots/README.md for the workflow.

Refs: AUDIT_APPSTORE.md P0-5

docs/screenshots/README.md

@@ -0,0 +1,73 @@
+# App Store Screenshots
+
+> ⚠️ **PLACEHOLDERS.** The 10 PNGs in this directory are
+> automatically generated blanks produced by
+> `scripts/generate_screenshot_placeholders.py` and must be
+> **replaced with real Simulator screenshots** before App Store
+> submission. They use the correct dimensions (1290×2796 for
+> 6.7", 1179×2556 for 6.1") so the upload validator will accept
+> them, but they contain no real UI.
+
+## Required dimensions (2026)
+
+| Size | Devices | Dimensions | Apple requirement |
+|------|---------|-----------|---|
+| 6.7" | iPhone 17 Pro Max, 17, 16 Pro Max, 16 Plus, 15 Pro Max, 15 Plus | 1290 × 2796 px | **Required** (3-10 images) |
+| 6.1" | iPhone 17 Pro, 17, 16 Pro, 16, 15 Pro, 15, 14 Pro, 14 | 1179 × 2556 px | **Required** (3-10 images) |
+| 5.5" | iPhone 8 Plus (legacy) | 1242 × 2208 px | Optional since 2024 |
+
+iPad screenshots are not required because OSGKeyboard is iPhone-only
+(`TARGETED_DEVICE_FAMILY = 1`).
+
+## Layout
+
+```
+docs/screenshots/
+├── 6.7/    ← 1290×2796 (iPhone 17 Pro Max / 17)
+│   ├── 01-keyboard-default.png
+│   ├── 02-flow-session.png
+│   ├── 03-on-device-asr.png
+│   ├── 04-llm-polish.png
+│   └── 05-providers.png
+└── 6.1/    ← 1179×2556 (iPhone 17 Pro / 17)
+    ├── 01-keyboard-default.png
+    ├── 02-flow-session.png
+    ├── 03-on-device-asr.png
+    ├── 04-llm-polish.png
+    └── 05-providers.png
+```
+
+## How to capture real screenshots
+
+1. Open `OSGKeyboard.xcodeproj` in Xcode 26+
+2. Run on **iPhone 17 Pro** simulator (6.1" set) and **iPhone 17 Pro Max** simulator (6.7" set)
+3. For each scene:
+   ```bash
+   # Take a screenshot of the simulator window
+   xcrun simctl io booted screenshot ~/Desktop/shot.png
+   ```
+4. Process for App Store (Apple rejects frames containing the device bezel — full-screen content only):
+   ```bash
+   # The simulator screenshot already has a thin device frame.
+   # Open in Preview, crop to full screen (⌘+K with ⌥ for precision),
+   # export as PNG at 1290×2796 or 1179×2556.
+   sips -z 2796 1290 shot.png --out final-6.7.png
+   sips -z 2556 1179 shot.png --out final-6.1.png
+   ```
+5. Replace the placeholders with the real captures, keeping the
+   same filenames so the App Store Connect → Version → Uploads UI
+   auto-pairs by file.
+
+## Scenes to capture
+
+The 5 placeholders are intentional scene placeholders. Capture these
+*exact* screens, in this order:
+
+1. **Keyboard at rest** — the iOS keyboard, OSGKeyboard mode, no recording
+2. **Flow session active** — keyboard with the green/orange recording ring,
+   partial transcript visible in the host text field
+3. **On-device ASR** — Settings view with the locale list, the on-device
+   indicator (iPhone icon) visible next to ≥ 3 supported locales
+4. **LLM polish** — Settings view with the API provider card, a sample
+   "polish" transformation shown in the inline preview
+5. **Providers** — API Settings card scrolled to show all 6 provider logos