feat: add per-hop pool references in listener chains

Allow listeners to mix named pools in a single chain using pool:name
syntax. Bare "pool" continues to use the listener's default pool.
Replaces pool_hops field with pool_seq list; pool_hops is now a
backward-compatible property. Each hop draws from its own pool and
failure reporting targets the correct source pool.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/USAGE.md

@@ -177,6 +177,36 @@ listeners:
       - pool
 ```
 
+### Per-hop pool references
+
+Use `pool:name` to draw from a specific named pool at that hop position.
+Bare `pool` uses the listener's `pool:` default. This lets a single listener
+mix pools in one chain.
+
+```yaml
+listeners:
+  - listen: 0.0.0.0:1080
+    pool: clean                              # default for bare "pool"
+    chain:
+      - socks5://10.200.1.13:9050
+      - pool:clean                           # explicit: from clean pool
+      - pool:mitm                            # explicit: from mitm pool
+
+  - listen: 0.0.0.0:1081
+    pool: clean
+    chain:
+      - socks5://10.200.1.13:9050
+      - pool                                 # bare: uses default "clean"
+      - pool:mitm                            # explicit: from mitm pool
+```
+
+| Syntax | Resolves to |
+|--------|-------------|
+| `pool` | Listener's `pool:` value, or `"default"` if unset |
+| `pool:name` | Named pool `name` (case-sensitive) |
+| `pool:` | Same as bare `pool` (empty name = default) |
+| `Pool:name` | Prefix is case-insensitive; name is case-sensitive |
+
 The `pool` keyword in a chain means "append a random alive proxy from the
 assigned pool". Multiple `pool` entries = multiple pool hops (deeper chaining).