feat(watchos): arm64_32 device support (Apple Watch Series 4-8 / SE)

[proggeramlug]

Why

Real Apple Watches before the Series 9 use the arm64_32 architecture (64-bit ISA, 32-bit pointers). Perry has only ever produced arm64 watch binaries (S9+/watchOS 26), and an internal note claimed arm64_32 was architecturally impossible because NaN-boxing needs a 64-bit usize.

That's overstated. A 32-bit pointer fits trivially in the 48-bit NaN payload; only a few heap-range guard heuristics and literal constants assumed a 64-bit address space. With those handled, the whole stack builds and links for arm64_32.

What

Runtime (32-bit literal fixes — also fix wasm32):

• array/from_concat: (1usize << 53) saturates via try_from(...).unwrap_or(usize::MAX) on 32-bit.
• box.rs, builtins/formatting.rs, value/dynamic_object.rs: the 2^47 / 2^48 pointer-guard upper bounds are compared in u64 so the literal isn't out-of-range for 32-bit usize (a no-op on 32-bit — no address reaches that high).
• The heap-min floor guards already select 0x1000 on target_os = "watchos", so no value changes were needed there.

Compile driver (opt-in via PERRY_WATCHOS_ARM64_32, leaving the default watchos=arm64/S9+ path untouched):

• Codegen emits the C entry directly via PERRY_ENTRY_SYMBOL (e.g. _perry_user_main) instead of renaming _main with rust-objcopy after the fact — objcopy's MachOWriter segfaults on arm64_32 Mach-O (both rustup- and Homebrew-shipped llvm-objcopy). The link-time objcopy rename is skipped on arm64_32.
• helpers.rs / app_metadata.rs / link/mod.rs / platform_cmd.rs / bundle_apple.rs resolve the arm64_32-apple-watchos triple consistently (codegen object arch, auto-optimize runtime rebuild, native-lib build, swiftc, final link) and a low MinimumOSVersion (PERRY_WATCHOS_MIN, default 11.0).

Validation

Built a real game (Bloom Jump) end to end:

• perry-runtime + perry-ui-watchos + the engine native lib all compile for arm64_32-apple-watchos (nightly -Z build-std).
• The game links to a Mach-O architecture: arm64_32, platform WATCHOS, minos 11.0 with __perry_user_main and _main both defined and no undefined refs.
• Host (aarch64) perry-runtime still builds clean — the runtime changes are no-ops on 64-bit.

Note: arm64_32 binaries cannot run on the watchOS simulator (the sim is arm64), so on-device validation requires real pre-S9 hardware; that test is pending.

Notes / follow-ups

• The opt-in is env-gated to keep the diff small and the arm64 path risk-free. A dedicated watchos-arm64_32 target name (or a fat arm64+arm64_32 build, which would let a single App Store upload cover Series 4 through 11 and drop the watchOS-27 minimum) is a natural follow-up — happy to take direction.
• arm64_32-apple-watchos is tier-3: runtime libs need cargo +nightly build -Z build-std. Wiring it into release-packages.yml would let PERRY_RUNTIME_DIR users skip the local build.