nx_channel.nx

channel.nx -- typed message-passing primitive (Phase M1 seed). Research: Honda 1993 ("Types for Dyadic Interaction"), Pony actor-channel model, Go channels with session-type extensions. The long-term destination is full session types (Honda 1998) where the channel's type describes the entire protocol; this file ships the foundation on which session-type checking will stack in a future parse.nx pass. Today's capabilities: * Typed FIFO channel with bounded capacity * Non-blocking try_send / try_recv * Blocking send / recv (spin-on-empty/full; MCU-friendly) * Single-producer single-consumer (SPSC); MPSC / MPMC variants ship in a follow-up once we have atomic primitives Currently specialised to i64 message type; generalises to Chan<T> when full multi-parameter generics land in parse.nx. Scaling claim: same channel.nx source compiles on MCU (no kernel threads, cooperative yield via __wfi between poll attempts) and on supercomputer (the scheduler's context switch uses the same primitive). Nothing about the API changes.