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>

README.md

@@ -93,6 +93,7 @@ proxy_pools:
     max_fails: 3
 
 # Multi-listener: each port gets a chain depth and pool assignment
+# Use "pool" for listener default, "pool:name" for explicit pool per hop
 listeners:
   - listen: 0.0.0.0:1080
     pool: clean
@@ -104,16 +105,11 @@ listeners:
     pool: clean
     chain:
       - socks5://127.0.0.1:9050
-      - pool                           # Tor + 1 clean proxy
+      - pool:clean                     # per-hop: clean pool
+      - pool:mitm                      # per-hop: mitm pool
   - listen: 0.0.0.0:1082
     chain:
       - socks5://127.0.0.1:9050       # Tor only
-  - listen: 0.0.0.0:1083
-    pool: mitm
-    chain:
-      - socks5://127.0.0.1:9050
-      - pool                           # Tor + 2 MITM proxies
-      - pool
 
 # Singular proxy_pool: still works (becomes pool "default")
 
@@ -150,9 +146,8 @@ Options:
 
 ```
 :1080  Client -> s5p -> Tor -> [clean] -> [clean] -> Dest  (2 clean hops)
-:1081  Client -> s5p -> Tor -> [clean] -> Dest              (1 clean hop)
+:1081  Client -> s5p -> Tor -> [clean] -> [mitm]  -> Dest  (mixed pools)
 :1082  Client -> s5p -> Tor -> Dest                         (Tor only)
-:1083  Client -> s5p -> Tor -> [mitm]  -> [mitm]  -> Dest  (2 MITM hops)
 ```
 
 s5p connects to Hop1 via TCP, negotiates the hop protocol (SOCKS5/4/HTTP),