P3 async intrinsic compilation: generate ARM code for stream/future/task host calls

[avrabe]

Context

When Meld lowers P3 async components to core modules, the output contains imports for RFC #46 host intrinsics (stream_read, stream_write, waitable_set_wait, task_return, etc). Synth must compile call sites for these intrinsics to native ARM code that invokes kiln-builtins, which dispatches to Gale's verified RTOS primitives.

What Synth Needs

1. Host Intrinsic Call Convention

RFC #46 intrinsics are imported as core WASM functions. Synth already handles host function imports by generating calls to kiln-builtins' C ABI surface. The P3 intrinsics follow the same pattern:

;; Core module (after Meld lowering)
(import "pulseengine:async" "stream_read" (func $stream_read (param i32 i32 i32 i32) (result i32)))

;; Synth generates ARM:
BL kiln_builtins_stream_read   ; Call into kiln-builtins

2. Callback Trampoline Code Generation

For P3 callback lifting mode, Meld generates a __callback export that the host calls. Synth compiles this export as a normal function entry point, but must ensure:

• The callback can be called from Gale's scheduler context (correct stack setup)
• Linear memory base register is restored on entry (callback may fire on a different Gale task)
• Fuel metering continues across callback invocations

3. Stream Buffer Memory Layout

Stream read/write intrinsics reference linear memory for data transfer. Synth must generate correct memory access patterns:

• Bounds check before stream_read writes to linear memory
• Alignment requirements for typed stream elements
• Integration with Synth's existing memory bounds checking proofs

4. Async State Preservation

If a component uses stackful async (not callback mode), Synth must generate code that:

• Saves register state before waitable_set_wait (which yields to Gale scheduler)
• Restores register state on resume
• Maintains correct stack frame across yield points

Verification

• Extend Z3 translation validation to cover host intrinsic call sites
• Verify register save/restore around yield points
• Verify memory bounds for stream buffer arguments

Connects to

• kiln#230: P3 async runtime (provides the intrinsics Synth calls)
• meld#94: P3 lowering (generates the intrinsic imports Synth compiles)
• gale#13: Typed stream extensions (provides the backend)
• synth#77: Platform abstraction (intrinsic calling convention per platform)

Priority

Medium-High — follows after Meld P3 lowering and Kiln P3 runtime.