docs(research): revise alknet-ssh phase-0 — channel decomposition, WebTransport grounding, WASM client
Reframes the SSH scope around the channel multiplexer as the decomposition point. Each feature (forwarding, SOCKS5, SFTP) is a channel type or a consumer of channel types, stacking on the core — each layer functional when built, none shipped broken. Dissolves the 'massive v1' framing that produced hedging language proposing non-functional or half-built versions. Three developments since the initial 2026-06-25 research changed the framing: (1) WebTransport landed as ADRs 038/040/043, grounding SSH-over-WebTransport as a constraint (the handler must be source-agnostic about its Connection); (2) russh's runtime abstraction (russh-util swaps tokio::spawn for wasm_bindgen_futures on wasm32) means the SSH *client* runs in WASM when fed a WebTransport BiStream — the browser case is real, not speculative; (3) the http crate intersection (ALPN-stream-proxy depends on SSH handlers being source-agnostic) is now visible and specified. The layered build order (1-4 stream+connection+channels+exec, then 5 forwarding, then 6 SOCKS5, then 7 SFTP) doubles as the configuration surface: each layer beyond the core is an opt-in channel type, gating on the default-deny ACL baseline inherited from russh.
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