docs(arch): ADR-057 — alknet-tty does not depend on alknet-call (self-contained negotiation framing)

The earlier specs claimed alknet-tty reuses alknet-call's
FrameFramedReader/FrameFramedWriter 'framing utility' (ADR-052 §6,
ADR-003 Am. 1). A pre-implementation sanity check found this was
unsound: FrameFramedReader::read_frame() is hardcoded to deserialize
EventEnvelope — the length-prefix read and the type-specific deserialize
are one entangled call, not a separable utility. alknet-tty's
negotiation payload is a NegotiateRequest, not an EventEnvelope, so the
claimed reuse did not exist in a usable form.

ADR-057: alknet-tty implements its own ~30-line length-prefixed
framing directly on tokio AsyncRead/AsyncWrite. The format coincides
with alknet-call's framing by convention (both are length-prefixed
JSON); the implementations are independent. alknet-tty depends on
alknet-core only — no alknet-call dependency. The ~30 lines of framing
is an idiom, not a domain abstraction worth a cross-crate dependency.

Updates:
- ADR-003 Amendment 2: clarifies alknet-tty does not depend on
  alknet-call; the Am. 1 protocol-foundation exception remains for
  alknet-http/agent/napi (actual type reuse), not alknet-tty.
- ADR-052 §6: framing is self-contained in alknet-tty; format coincides
  by convention, not by code reuse.
- overview.md: dependency block drops alknet-call; new 'Why no
  alknet-call dependency' subsection explains the unsound-reuse finding.
- tty-wire.md, tty-backend.md, tty README, crates/tty/README.md,
  top-level README: ADR tables, Design Decisions tables, and dependency
  edges updated to reflect alknet-core-only dependency.
- ADR-053, ADR-054: References updated to ADR-003 Am. 2 + ADR-057.

docs/architecture/decisions/054-local-tty-backend-sibling-crate.md

@@ -191,7 +191,7 @@ is NOT" — door type classifies reversal cost, not urgency.
 
 - `docs/research/alknet-tty/phase-0-findings.md` DP-1 — the placement
   question this ADR resolves
-- [ADR-003](003-crate-decomposition.md) + Amendment 1 — crate
+- [ADR-003](003-crate-decomposition.md) + Amendments 1 & 2 — crate
   decomposition rule (the sibling-crate placement preserves it)
 - [ADR-009](009-one-way-door-decision-framework.md) — door-type-as-deferral
   anti-pattern (this ADR's two-way-door classification is reversal cost,