feat(bindgen): per-function async override for p3 (async_interfaces) (#526)

[avrabe]

Fixes #526.

Problem

rust_wasm_component_bindgen(wasi_version = "p3") hardcoded --async all, so every export — including functions whose WIT signature is plain sync — was lifted async ([async-lift]… / [task-return]…). A call-return consumer can't invoke an async-lift export, so relay had to stand up a whole separate p2 sibling component just to expose one sync entry point for its witness MC/DC harness.

Change

Adds an async_interfaces parameter to rust_wasm_component_bindgen exposing wit-bindgen's --async filters on the guest bindings (native-guest stays sync — the host has no async_support). Default is unchanged:

wasi_version	default async_interfaces
p3	["all"]
p2	[]

Callers can now mix sync and async exports:

async_interfaces = []                                       # WIT defaults: async-typed → async, sync → sync
async_interfaces = ["-all"]                                 # force every export sync
async_interfaces = ["-export:pkg:iface/i@0.1.0#fn"]         # that export sync, others WIT-default
async_interfaces = ["pkg:iface/i@0.1.0#stream-fn"]          # that export async, others WIT-default

For #526's exact case — keep stream exports async, lift one function sync — the cleanest answer is often async_interfaces = []: once the WIT distinguishes async-typed (stream/future) from sync functions, wit-bindgen's per-function default already does the right thing; it was only the blanket all forcing the sync ones async.

Also fixes _build_async_args to emit --async=VALUE instead of --async VALUE, so filter values starting with - (-all, -export:…) aren't misparsed by clap as separate flags (error: unexpected argument '-e').

Verification

New //test/p3:hello_p3_sync builds a p3 component whose greet export is implemented as a plain sync fn greet (no async, no #[cfg] split) using async_interfaces = ["-export:hello:interfaces/greeting@0.1.0#greet"]:

• ✅ bazel test //test/p3:p3_sync_export_build_test passes.
• 🔬 Falsification: under the p3 default (--async=all) the same sync impl fails to compile — method should be async or return a future, but it is synchronous — and the generated bindings carry 3 async-lift markers for greet; with the override they carry 0.
• ✅ Default p3 path (//test/p3:hello_p3, --async=all) still builds — no regression from the arg-form change.
• ✅ p2_build_test, p3_build_test still pass.

wit-bindgen 0.54 contract (documented in the rule)

• Filter names must include the package version (pkg:iface/i@0.1.0#fn).
• Combining the all blanket with a per-export -export: exclude is rejected (unused async option) — use [] (WIT defaults) or the allowlist form instead.

Refs: pulseengine/relay#202, pulseengine/witness#107

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