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).

mobile/ios/README.md

@@ -1,5 +1,25 @@
 # iOS build (LÖVE 11.5)
 
+> **Native ROM/mod/save import.** The iOS build ships a Swift
+> document-picker bridge (`native/GRPickerBridge.swift` + `GRBootstrap.m`)
+> that `patch_love_src.py` wires into the LÖVE tree on every build:
+>
+> - `love.system.pickFile("rom"|"mod"|"sav")` and `love.system.createFile`
+>   are exposed to Lua on iOS (same contract as love-android's SAF picker:
+>   picks land in the save dir as `picked_rom.gb` / `picked_mod.zip` /
+>   `picked_save.sav`; exports signal via `export_done.flag`).
+> - The Info.plist overlay enables `UIFileSharingEnabled` +
+>   `LSSupportsOpeningDocumentsInPlace`, and `GRBootstrap.m` sweeps
+>   `.gb/.gbc/.zip/.sav` files dropped in Documents (Files app / Finder)
+>   into the LÖVE save dir on every activation — drop a ROM, open the app,
+>   and it imports with no taps.
+> - `src/import/RomImporter.lua` treats iOS as a mobile platform and polls
+>   for picker results (iOS pickers are in-process modals, so Android's
+>   refocus rescan never fires).
+>
+> The note below about a missing "UIDocumentPicker handoff" is
+> resolved by this bridge.
+
 macOS + Xcode only. Pins the official **LÖVE 11.5** iOS Xcode tree
 (`love-11.5-ios-source.zip` from [love2d/love releases](https://github.com/love2d/love/releases/tag/11.5)),
 matching `conf.lua`'s `t.version = "11.5"`.