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/overview.md

@@ -217,6 +217,7 @@ All design decisions are documented as ADRs in [decisions/](decisions/).
 | [024](decisions/024-operation-registry-layering.md) | Operation Registry Layering | Curated (static) + session/connection overlays (dynamic); `OperationEnv` as trait-object integration point |
 | [025](decisions/025-vault-local-only-dispatch.md) | Vault Local-Only Dispatch | Dropped irpc from vault; direct method calls; local-only by construction |
 | [026](decisions/026-vault-key-model-hd-derivation.md) | Vault Key Model — HD Derivation | HD derivation from BIP39 seed; `74'` coin type; SLIP-0010/Ed25519 default; AES-256-GCM for credentials |
+| [027](decisions/027-tls-identity-redesign-acme-rawkey-decoupling.md) | TLS Identity Redesign — ACME + RawKey Decoupling | `TlsIdentity::Acme` variant + two-phase server config; `RawKey` uses `ed25519-dalek` (not `iroh::SecretKey`); `acme` feature gate |
 
 ## Open Questions