docs(adr-027): TLS identity redesign — ACME + RawKey decoupling

ADR-027 resolves the architectural gap surfaced when ACME integration
became a concrete target:

1. TlsIdentity::Acme variant — static config data (domains, cache_dir,
   directory, contact) with async AcmeState constructed at endpoint
   setup via two-phase TlsSetup (not stuffed into the Clone-able enum).

2. TlsIdentity::RawKey decoupled from the iroh feature — uses
   Ed25519SecretKey (alknet-core-owned wrapper over ed25519_dalek)
   instead of iroh::SecretKey. Raw-key TLS identity (RFC 7250, the
   default for most alknet nodes) now works in quinn-only builds.
   iroh transport converts via SecretKey::from_bytes.

3. ACME feature-gated behind new acme feature (rustls-acme optional
   dep). Non-ACME builds don't compile it.

4. dispatch_quinn guard for acme-tls/1 challenge connections — TLS-ALPN-01
   is handled at the rustls cert resolver layer during the handshake;
   the guard closes challenge connections gracefully instead of logging
   a misleading "no handler" warning.

Research confirmed QUIC (quinn) handles ACME challenges differently than
TCP (reverse-proxy): quinn gives no ClientHello peek hook, but the
challenge is fully answered at the cert resolution step before the
connection surfaces to the application. No handler registration needed.

Spec updates: config.md, endpoint.md, open-questions.md (OQ-12),
overview.md + README.md (ADR index), ADR-010 (cross-ref).

Tasks: core/rawkey-decouple-from-iroh (gen 1, no deps),
core/acme-integration (gen 2, depends on rawkey). Graph: 36 tasks.

docs/architecture/decisions/010-alpn-router-and-endpoint.md

@@ -183,7 +183,7 @@ pub enum HandlerError {
 **Negative:**
 - alknet-core depends on both quinn and iroh (mitigated: both are feature-gated; a node that only needs one doesn't compile the other)
 - The endpoint is more complex than a single quinn listener — it manages multiple accept loops
-- TLS identity provisioning has two distinct use cases: RFC 7250 raw keys (default for P2P/key-based identity) and X.509 certs (for domain-hosted services and browsers). ACME auto-provisioning for X.509 is a proven pattern from the reverse-proxy project, not speculative future work. See OQ-12.
+- TLS identity provisioning has two distinct use cases: RFC 7250 raw keys (default for P2P/key-based identity) and X.509 certs (for domain-hosted services and browsers). ACME auto-provisioning and RawKey decoupling from the `iroh` feature are designed in ADR-027. See OQ-12.
 - No runtime handler registration without regenerating the TLS config (mitigated: two-way door, start static, add ArcSwap later if needed)
 
 ## References