feat(plugin-wasm): migrate plugin system to WASI 0.3 / component-model async

[staging-devin-ai-integration]

Summary

• Migrates the WASM plugin system to WASI 0.3 / Component Model native async in a single lane (no 0.2 compatibility world, per discussion): process, update-params, and the send-output host import are now async func in wit/plugin.wit@0.2.0.
• Host moves to wasmtime 45 with component_model_async: guest calls run inside store.run_concurrent, and async imports are implemented via the generated HostWithStore trait. This unblocks future concurrent/cancellable guest calls and stream<T> pins.
• The WIT deps are vendored from the exact wasi:cli@0.3.0-rc-2026-03-15 snapshot wasmtime 45's p3 host links against (final 0.3.0 WITs aren't in wasmtime yet). The linker registers both p2 and p3 WASI because Rust's wasm32-wasip2 std still imports wasi@0.2 interfaces.
• Rust SDK regenerated with wit-bindgen 0.58 (async guest bindings; runtime now lives in wit_bindgen::rt, replacing the dead wit-bindgen-rt crate); guests build with plain cargo build --target wasm32-wasip2 — cargo-component and wkg are no longer needed.
• C SDK + gain example migrated to the async callback ABI (subtask parked in a waitable set when send-output doesn't complete synchronously). Go is left behind for now: wit-bindgen-go cannot parse async WIT yet, so the Go example/bindings stay on the old world with a README notice and the just recipe disabled.

Review & Validation

• cargo test -p streamkit-plugin-wasm --test gain_plugin_smoke -- --ignored passes after building the Rust (just build-plugin-wasm-rust) and C (just build-plugin-wasm-c, needs wasi-sdk) examples — drives a real component through the full async host path (construct → process → routed output → shutdown).
• Sanity-check the dual p2+p3 linker setup in crates/plugin-wasm/src/lib.rs and the run_concurrent loop in wrapper.rs for cancellation/shutdown behavior.
• Existing oneshot sample samples/pipelines/oneshot/gain_filter_rust.yml still exercises the plugin end-to-end via tests/pipeline-validation.

Notes

• wasmtime's p3 host is still documented as experimental; we accepted that risk explicitly given v0.x status and no real-world WASM plugins yet.
• Follow-ups: re-enable Go bindings once wit-bindgen-go gains component-model async support; consider stream<packet>-shaped pins as a next step.

Link to Devin session: https://staging.itsdev.in/sessions/8a4721b3ccbb4a4d9ea85422ade86fdf
Requested by: @streamer45

---

Devin Review

Status	Commit
🟢 Reviewed	9a6cd73

[staging-devin-ai-integration]

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[staging-devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

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[codecov]

Codecov Report

❌ Patch coverage is 38.70968% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.27%. Comparing base (1b95ca2) to head (9a6cd73).

Files with missing lines	Patch %	Lines
crates/plugin-wasm/src/lib.rs	48.00%	13 Missing ⚠️
crates/plugin-wasm/src/wrapper.rs	0.00%	6 Missing ⚠️

❌ Your patch check has failed because the patch coverage (38.70%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #588      +/-   ##
==========================================
- Coverage   82.43%   82.27%   -0.17%     
==========================================
  Files         240      240              
  Lines       71297    71416     +119     
  Branches     2249     2249              
==========================================
- Hits        58773    58755      -18     
- Misses      12520    12657     +137     
  Partials        4        4              

Flag	Coverage Δ
backend	82.23% <38.70%> (-0.18%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
core	85.67% <ø> (ø)
engine	83.33% <ø> (-0.09%)	⬇️
api	91.14% <ø> (ø)
nodes	82.78% <ø> (-0.05%)	⬇️
server	80.52% <ø> (ø)
plugin-native	83.89% <ø> (ø)
plugin-wasm	82.49% <38.70%> (-9.72%)	⬇️
ui-services	85.04% <ø> (ø)
ui-components	64.05% <ø> (ø)

Files with missing lines	Coverage Δ
crates/plugin-wasm/src/wrapper.rs	66.02% <0.00%> (-30.35%)	⬇️
crates/plugin-wasm/src/lib.rs	86.35% <48.00%> (-2.09%)	⬇️

... and 2 files with indirect coverage changes

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