docs(architecture): resolve one-way doors, clean up Phase 0 specs

Resolve blocking one-way door decisions:
- ADR-007: BiStream is a trait, handlers receive Connection not BiStream
- ADR-008: Secret service is CLI-embedded, exposed via call protocol
- ADR-009: One-way door decision framework (classify by reversal cost)

Update existing documents:
- overview.md: add design principles, revise ProtocolHandler signature,
  update shared types, add WASM as design constraint
- open-questions.md: add door-type classifications, resolve OQ-01/OQ-08,
  move OQ-09/OQ-10 to deferred section, mark two-way doors as impl-deferred
- README.md: reflect resolved questions, remove crate spec stubs from index
- ADR-002: cross-reference ADR-007 for signature revision

Clean up premature artifacts:
- Remove 11 empty crate spec stubs (16-28 lines each, no unique content)
- Specs will be created when each crate enters Phase 1

docs/architecture/decisions/002-protocol-handler-trait.md

@@ -53,5 +53,6 @@ pub trait ProtocolHandler: Send + Sync + 'static {
 - Pivot proposal: `docs/research/pivot/alpn-service-architecture.md`
 - ADR-001: ALPN-based protocol dispatch
 - ADR-004: Auth as shared core (IdentityProvider)
+- ADR-007: BiStream type definition (revised this ADR's signature from BiStream to Connection)
 - iroh ProtocolHandler pattern: `docs/research/references/iroh/`
 - Replaces StreamInterface, MessageInterface, and ListenerConfig