P3 async — cross-component stream adapter (#94 sub-A)

[avrabe]

Sub-issue of #94 — outstanding deliverable A from the audit.

Today meld lowers each component's stream intrinsics to host imports under `pulseengine:async`. When two fused components share a `stream` endpoint, those calls still go through the host even when both sides live in the merged module. The umbrella asks for a stream-bridging adapter that fuses both ends.

Scope

Two modes, mirroring the existing multi-memory / shared-memory split for synchronous data:

Mode	Adapter shape
Same memory (single-memory fusion)	Direct ring buffer — sender and receiver share an in-module queue page; zero-copy.
Different memories (multi-memory, default)	`stream_read` → in-module copy loop → `stream_write` chain. `cabi_realloc`-style allocation on the receiver side, identical null-guard policy as LS-A-7.

Deliverables

• New emitter in `adapter/fact.rs` (or a sibling file) that detects intra-fusion `stream` pairs at merge time and emits the adapter.
• Resolver extension: track `(source_component, source_stream_handle) → (sink_component, sink_stream_handle)` pairs.
• Tests: same-memory zero-copy, different-memory chain, backpressure propagation through the adapter, EOF passthrough.
• Mythos check on the new emitter (same Tier-5 process as fact.rs).
• ADR-3: cross-component stream adapter design.

Dependencies

• None blocking. Builds on the foundation in feat: P3 async lowering — foundation + stream<T> e2e (partial #94) #124 / feat: P3 async lowering pass — canon stream/future → pulseengine:async (closes #120) #129. Sub-B static validation (#TBD) wants this landed first to have a thing to validate.

Rivet artifact

• New requirement SR-33 — "Cross-component `stream` fusion at merge time".

Milestone

v0.9.0