feat: round-6 abstractions — HealthCheckedPool, HkdfKeyDeriver, BackpressureEnforcer

.hermes/plans/mytheclipse-round6-spec.md

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+# Implementation Spec: Round 6
+
+## New Features
+
+### 1. HealthCheckedPool (mytheclipse-core, observability+traffic)
+File: `crates/mytheclipse/src/pool_health.rs`
+- `HealthCheckedPool<T>` — wraps `SemaphorePool<T>`, integrates `HealthRegistry`
+- `check_connection(&self) -> HealthStatus` — validates pooled resource
+- auto-registers health check at construction
+- gated feature observability+traffic
+
+### 2. HkdfKeyDeriver (mytheclipse-crypto, derivation feature)
+File: `crates/mytheclipse-crypto/src/hkdf.rs`
+- `HkdfKeyDeriver` — HKDF-SHA256 (RFC 5869) from master secret
+- `derive_key(&self, purpose: &str, output_len) -> Vec<u8>` — context-specific sub-key
+- domain separation via purpose as info
+- gated feature "derivation"
+
+### 3. BackpressureEnqueue (mytheclipse-queue, in-memory)
+File: `crates/mytheclipse-queue/src/backpressure.rs`
+- `BackpressureEnforcer` — tracks in-flight count, enforces max
+- `enqueue_or_nack(queue, topic, payload, max_inflight) -> Result<(), BackpressureError>`
+- non-blocking: returns BackpressureError when at capacity
+
+## Verification
+- build + test + clippy + commit + push