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>

config/example.yaml

@@ -87,30 +87,35 @@ chain:
 # a random alive proxy from the named pool (or "default" if unnamed).
 # Multiple "pool" entries = multiple pool hops (deeper chaining).
 #
+# Per-hop pool references: use "pool:name" to draw from a specific pool
+# at that hop position. Bare "pool" uses the listener's "pool:" default.
+# This lets a single listener mix pools in one chain.
+#
 # listeners:
 #   - listen: 0.0.0.0:1080
-#     pool: clean                        # draw from "clean" pool
+#     pool: clean                        # default for bare "pool"
 #     chain:
 #       - socks5://127.0.0.1:9050
-#       - pool                           # Tor + 2 clean pool proxies
-#       - pool
+#       - pool:clean                     # explicit: from clean pool
+#       - pool:mitm                      # explicit: from mitm pool
 #
 #   - listen: 0.0.0.0:1081
 #     pool: clean
 #     chain:
 #       - socks5://127.0.0.1:9050
-#       - pool                           # Tor + 1 clean pool proxy
+#       - pool                           # bare: uses default "clean"
+#       - pool
 #
 #   - listen: 0.0.0.0:1082
 #     chain:
 #       - socks5://127.0.0.1:9050       # Tor only (no pool hops)
 #
 #   - listen: 0.0.0.0:1083
-#     pool: mitm                         # draw from "mitm" pool
+#     pool: clean
 #     chain:
 #       - socks5://127.0.0.1:9050
-#       - pool                           # Tor + 2 MITM pool proxies
-#       - pool
+#       - pool                           # bare "pool" = clean
+#       - pool:mitm                      # explicit = mitm
 #
 # When using "listeners:", the top-level "listen" and "chain" keys are ignored.
 # If "listeners:" is absent, the old format is used (single listener).