iOS: native document-picker + Apple Health bridges, working device builds

Makes the iOS build a first-class citizen: ROM/mod/save import through
the system document picker (the README's missing "UIDocumentPicker
handoff"), Files-app drop-in support, and an opt-in Apple Health
step-sync seam consumed by a new gallery mod (Pokewalker).

Native layer (mobile/ios/native/, wired by mobile/ios/patch_love_src.py
on every build, so the fetched love-src tree stays pristine + re-patchable):
- GRPickerBridge.swift: love.system.pickFile("rom"|"mod"|"sav") and
  love.system.createFile on iOS with the same contract as love-android's
  SAF picker (picked_rom.gb / picked_mod.zip / picked_save.sav /
  export_done.flag in the save dir). Reached from wrap_System.cpp via the
  ObjC runtime, so liblove needs no Swift interop.
- GRBootstrap.m: sweeps .gb/.gbc/.zip/.sav dropped in Documents (Files
  app / Finder sharing) into the save dir on every activation;
  UIFileSharingEnabled + LSSupportsOpeningDocumentsInPlace in the plist
  overlay. Drop a ROM, open the app, it imports with zero taps.
- GRHealthBridge.swift: love.system.syncHealthSteps() -> read-only
  HealthKit step query anchored to the last sync, delivered as
  steps_pending.json (merge-not-overwrite). HealthKit entitlement +
  usage description included.

Lua:
- RomImporter: iOS rides the Android mobile flows; a 0.5s poll consumes
  picker deliveries (iOS pickers are in-process modals, so the Android
  refocus rescan never fires); failed pick copies surface as an
  on-screen notice via pick_error.txt.
- main.lua: on iOS, stop forwarding touchpressed to the Importer - LOVE
  already synthesizes a mousepressed for the primary touch, and the
  resulting same-frame double-present made the document picker
  auto-dismiss with zero documents (silent import failure).
- mods/pokewalker: opt-in Pokewalker mod (manifest v2, MECHANIC,
  permissions declared, mod.card, CHANGELOG, headless test suite 9/9,
  modkit validate --base imported + lint clean). Fused into iOS
  game.love only; loads dormant anywhere without the bridge.

Build (scripts/build_ios.sh):
- Fix Xcode 26: the global PRODUCT_NAME override also renamed liblove.a
  and broke the app link; the app bundle is renamed after the build
  instead.
- Fix nondeterministic pack failures: grep -q + pipefail races SIGPIPE
  on the game.love content checks.
- Simulator builds sign ad-hoc so entitlements embed (HealthKit works in
  the simulator).
- Device builds: signing team auto-detected from the keychain,
  CODE_SIGN_STYLE=Automatic + -allowProvisioningUpdates for CLI-only
  provisioning, per-team derived bundle ID (explicit App IDs are
  globally unique, so third parties can't sign the project default),
  gitignored mobile/ios/bundle_id.local pin, and --install to push to a
  connected iPhone.
- docs/ios-install.md: a zero-knowledge walkthrough from bare Mac to
  playing on an iPhone.

Backward compatibility: no behavior change on desktop or Android. The
new love.system functions exist only under LOVE_IOS; RomImporter's
mobile flag simply includes iOS alongside Android; the main.lua change
is iOS-gated; the Pokewalker mod is packed only by the iOS build script
and its option defaults off.

Verified on an iPhone 17 Pro simulator and an iPhone 16 Pro device:
scripted ROM import to title screen, Files-drop zero-tap import,
picker-driven mod install and save import/export, HealthKit permission
sheet + step credit (4000 steps -> +200 EXP at the default rate through
the engine growth curve).

docs/ios-install.md

@@ -0,0 +1,116 @@
+# Build & install on your iPhone — step by step
+
+This guide assumes **zero** programming experience. Follow it top to
+bottom and you'll have the game running on your own iPhone in roughly an
+hour (most of it is waiting for downloads).
+
+## What you need
+
+- A **Mac** (any Apple-silicon or recent Intel Mac on macOS 14 or newer)
+- An **iPhone** and its **charging cable**
+- A free **Apple ID** (the same account you use for the App Store)
+- Your **own, legally obtained** Pokémon Red or Blue ROM file (a 1 MB
+  `.gb` file). This project ships no game data — the app rebuilds
+  everything from *your* cartridge dump and verifies it before use.
+- About **15 GB free disk space** (Xcode is enormous; the game itself is
+  tiny)
+
+> **Free vs. paid Apple account:** a free Apple ID works. The only catch:
+> apps signed with a free account stop launching after **7 days** — just
+> re-run the install command (step 5) to re-sign; your save data is kept.
+> A paid Apple Developer account ($99/yr) extends that to a year.
+
+---
+
+## Step 1 — Install Xcode (once)
+
+1. On the Mac, open the **App Store**, search **Xcode**, click **Get**.
+   It's a ~10 GB download — go make tea.
+2. Open Xcode once. Accept the license. If it offers to install extra
+   components or the **iOS platform**, say yes and let it finish.
+
+## Step 2 — Sign Xcode into your Apple ID (once)
+
+1. In Xcode's menu bar: **Xcode → Settings → Accounts**.
+2. Click the **+** in the bottom-left → **Apple Account** → sign in.
+3. Close the window. (This quietly creates the "signing certificate" the
+   build uses — you never have to touch it again.)
+
+## Step 3 — Get this project onto the Mac
+
+If you received it as a folder, put it somewhere easy like your home
+folder. If it's on GitHub, click the green **Code** button → **Download
+ZIP**, then double-click the zip to unpack it.
+
+## Step 4 — Build it (one command)
+
+1. Open the **Terminal** app (press ⌘-space, type `terminal`, press
+   return).
+2. Type `cd ` (c, d, space — don't press return yet), then **drag the
+   project folder** from Finder onto the Terminal window — it fills in
+   the path — and press **return**.
+3. Paste this and press return:
+
+   ```sh
+   scripts/build_ios.sh --fetch
+   ```
+
+   The first run downloads the LÖVE engine and compiles everything
+   (5–15 minutes). Lines of build output scrolling by is normal. You're
+   done when you see **`==> done`**.
+
+## Step 5 — Put it on your iPhone
+
+1. Plug the iPhone into the Mac with the cable. **Unlock it** and keep it
+   unlocked. If it asks **"Trust This Computer?" → Trust**.
+2. Paste this and press return:
+
+   ```sh
+   scripts/build_ios.sh --device --install
+   ```
+
+   The script finds your signing identity and your phone by itself. If it
+   complains, it tells you exactly what to fix (usually: the phone was
+   locked — unlock and re-run).
+
+3. First time only, the iPhone will want two approvals:
+   - **Developer Mode**: Settings → **Privacy & Security** → scroll to
+     **Developer Mode** → turn on → restart the phone → confirm.
+   - **Trust the developer**: Settings → **General** → **VPN & Device
+     Management** → tap the entry under *Developer App* → **Trust**.
+
+   Then re-run the command in step 5.2 if the install had failed.
+
+## Step 6 — Give it your ROM and play
+
+1. Get your `.gb` file onto the phone — AirDrop it to yourself, or save
+   it in iCloud Drive / Files.
+2. Open the app. On the **RED** tab, tap **Import ROM** and pick your
+   `.gb` file in the file browser that appears. (Alternative: in the
+   **Files** app, drop the `.gb` into **On My iPhone → the game's
+   folder** and just reopen the app — it imports automatically.)
+3. Import takes ~10 seconds, the button turns into **Play Red** — tap it.
+
+## Optional goodies
+
+- **Pokéwalker mode** (real steps → EXP): in-game **mod manager →
+  POKEWALKER → SYNC STEPS on**, allow step access when iOS asks, and go
+  for a walk.
+- **Mods**: launcher → **MODS** tab → **Import mod .zip**.
+- **Save import/export**: buttons on each game's tab, using the normal
+  iOS file picker.
+
+## When things go wrong
+
+| Symptom | Fix |
+|---|---|
+| `xcodebuild not found` | Xcode isn't installed or wasn't opened once — do Step 1 |
+| `no Apple signing identity found` | Do Step 2, then re-run |
+| `no iPhone/iPad found` | Cable in? Phone unlocked? Tapped "Trust"? |
+| Install fails with "locked" | Unlock the phone, keep it unlocked, re-run |
+| App icon appears then won't open | Do the two approvals in Step 5.3 |
+| App stops launching after a week | Free-account 7-day limit — re-run Step 5.2 |
+| "That ROM could not be imported" | The file isn't a canonical 1 MB US Red/Blue dump — the importer checks its fingerprint |
+
+Every build/install command is safe to re-run; your saves live on the
+phone and survive reinstalls.