docs(architecture): add Phase 0 architecture specs for ALPN-as-service model

Foundational architecture documents following the SDD process:

ADRs:
- 001: ALPN-based protocol dispatch (one endpoint, ALPN negotiation)
- 002: ProtocolHandler trait (replaces StreamInterface/MessageInterface)
- 003: Crate decomposition (one crate per handler, core provides shared infra)
- 004: Auth as shared core (IdentityProvider, hybrid resolution model)
- 005: irpc as call protocol foundation
- 006: ALPN string convention and connection model (alknet/ prefix, one ALPN per connection)

Docs:
- overview.md: crate graph, shared types, ALPN registry, failure modes
- README.md: index with doc table, ADR table, lifecycle definitions
- open-questions.md: 10 OQs across 7 themes (3 resolved, 7 open)

Crate spec stubs for all 11 planned crates (alknet-core through alknet CLI).

Key decisions resolved during self-review:
- AuthContext resolution is hybrid: endpoint resolves TLS-level auth,
  handlers resolve protocol-level auth (resolves OQ-02)
- ALPN is per-connection not per-stream, corrected ADR-001 (resolves OQ-06)
- ALPN naming uses alknet/ prefix without versions (resolves OQ-03)
- HandlerError return type on ProtocolHandler trait
- alknet/secret removed from ALPN registry until OQ-08 resolved

docs/architecture/crates/alknet-http/spec.md

@@ -0,0 +1,23 @@
+---
+status: planned
+last_updated: 2026-06-15
+---
+
+# alknet-http
+
+> **Status: Planned** — This spec has not been written yet.
+
+## Purpose
+
+HTTP handler implementing `ProtocolHandler` on ALPN `alknet/http`. Provides axum router with auth middleware, REST API, dashboard, and MCP endpoint. Also handles standard HTTP ALPNs (`h2`, `http/1.1`) and WebTransport upgrade on `h3`.
+
+## Key Questions
+
+- How does HttpAdapter handle both `alknet/http` and standard ALPNs (`h2`, `http/1.1`, `h3`)?
+- WebTransport upgrade on `h3` — is this a separate handler or integrated into HttpAdapter?
+
+## References
+
+- [overview.md](../../overview.md)
+- ADR-002: ProtocolHandler trait
+- ADR-006: ALPN string convention and connection model