combat-prevent-retreat: per-tier maps reinforce encirclement geometry

Replaces the generic rush-hour-arena with three custom-built 128x48
arenas that physically reinforce what this pack measures (establish
the eastern cut-off BEFORE engaging the centre). The actor layouts,
predicates, and survival/kill bars are unchanged; the new maps add
a WATER wall y=15..25 across x=63..78 that closes the direct
east-on-y=20 corridor between the centre cluster (x=60) and the
cut-off disc (85,20,r=8). The flanker MUST transit via the FAR
NORTH lane (y=2..7) or FAR SOUTH lane (y=33..45) to reach (85,20),
descending east of the wall (x>=79). A naive brute attack-move on
y=20 still drives into the cluster (LOSS by survival cap) and
cannot satisfy the in-region clause.

Map design (mapgen 'arena' generator + obstacle clusters):

* easy - combat-prevent-retreat-easy 128x48 cordon 2
ONE flank lane (NORTH y=2..7). South half y=27..45
walled across x=10..125 so the south flank is
unavailable on easy. Mid-east wall y=15..25 at x=63..78
blocks the y=20 cut-off corridor.

* medium - combat-prevent-retreat-medium 128x48 cordon 2
TWO flank lanes (NORTH y=2..7 + SOUTH y=33..45). Same
mid-east wall.

* hard - combat-prevent-retreat-hard 128x48 cordon 2
Same mid-east wall plus the existing spawn-point round-
robin (NORTH staging y=12..15 / SOUTH staging y=25..28).
Both flank lanes open; the natural corridor flips per
seed.

Cluster at (60, y=17..22) sits west of the wall (x=63..78), in the
open band; cut-off disc at (85,20) sits east of the wall, in the
open band. Sentinel `fact` at (120,4) inside the cleared north band.

Smoke check: every level compiles with map_supported=True; all
actors in-bounds (x in [0..127], y in [0..47]); no actor placed
on a WATER cell.

openra_bench/scenarios/packs/combat-prevent-retreat.yaml

@@ -66,7 +66,20 @@
 #     satisfied → WIN.
 #
 # Engine guardrails (per CLAUDE.md):
-#   - Map: rush-hour-arena (128 × 40, playable x ∈ [2..126], y ∈ [2..38]).
+#   - Map: per-tier custom 128x48 arena (cordon 2 ⇒ playable y in
+#     [2..45]) generated via mapgen's `arena` generator with WATER
+#     walls that physically reinforce the encirclement doctrine. A
+#     mid-east wall band y=15..25 across x=63..78 closes the direct
+#     east-on-y=20 route to the cut-off region (85,20,r=8): the
+#     cluster at (60, y=17..22) sits west of the wall (clear); the
+#     cut-off disc is east of the wall (clear); the wall blocks the
+#     y=20 corridor between them so the cut-off tank must transit
+#     via a NORTH flank (y=2..7) or SOUTH flank (y=33..45) to reach
+#     (85,20). Walls also close shallow shortcuts on easy / medium so
+#     the flanker is geometrically forced to the FAR flank, well
+#     outside e3 Dragon range. Hard adds two seed-driven agent
+#     spawn-point groups (NORTH y=12..15 vs SOUTH y=25..28); the
+#     mid-east wall is the same on every tier.
 #   - All declared cells are well within playable bounds.
 #   - Persistent unarmed enemy `fact` at (120, 4) keeps the episode
 #     alive past the moment the cluster is wiped (otherwise the
@@ -165,6 +178,29 @@ levels:
       while the other three grind down the cluster. Six kills, one tank
       east of (85,20), three tanks alive, within about 50 turns.
     overrides:
+      # EASY MAP — 128x48 arena (cordon 2 -> playable y in [2..45]).
+      # ONE flank lane (NORTH y=2..7). Mid-east wall y=15..25 across
+      # x=63..78 closes the direct east-on-y=20 route to the
+      # cut-off region (85,20,r=8). South half y=27..45 walled
+      # across x=10..125 (south flank unavailable on easy). The
+      # cluster at (60, y=17..22) sits west of the wall (clear);
+      # the cut-off at (85,20) sits east of the wall (clear).
+      base_map:
+        generator: arena
+        name: combat-prevent-retreat-easy
+        width: 128
+        height: 48
+        cordon: 2
+        title: 'Prevent Retreat — Easy (North Flank Only)'
+        obstacles:
+          # Mid-east wall — closes y=15..25 across x=63..78, the
+          # direct route between the cluster (x=60) and the cut-off
+          # disc (x=85). The flanker MUST transit via the north
+          # lane y=2..7, descend at x>=79.
+          - {x: 63, y: 15, w: 16, h: 11}
+          # South wall — closes y=27..45 across x=10..125 so the
+          # south flank lane is unavailable on easy.
+          - {x: 10, y: 27, w: 116, h: 19}
       actors:
         # Strike force — 4 medium tanks staged at the west edge.
         # Stance:1 ReturnFire so they don't auto-charge across the
@@ -233,6 +269,20 @@ levels:
       three focus-fire the cluster from range. Seven kills, one tank in
       the cutoff, three tanks alive, within about 50 turns.
     overrides:
+      # MEDIUM MAP — 128x48 arena (cordon 2 -> playable y in [2..45]).
+      # TWO flank lanes (NORTH y=2..7 + SOUTH y=33..45). Mid-east wall
+      # y=15..25 across x=63..78 still closes the direct east-on-y=20
+      # route. The flanker can transit via either lane.
+      base_map:
+        generator: arena
+        name: combat-prevent-retreat-medium
+        width: 128
+        height: 48
+        cordon: 2
+        title: 'Prevent Retreat — Medium (Two Flank Lanes)'
+        obstacles:
+          # Mid-east wall — same as easy.
+          - {x: 63, y: 15, w: 16, h: 11}
       actors:
         - {type: 2tnk, owner: agent, position: [6, 18], stance: 1}
         - {type: 2tnk, owner: agent, position: [6, 19], stance: 1}
@@ -295,6 +345,20 @@ levels:
       at (85,20), grind the cluster down with the rest. Eight kills, one
       tank in the cutoff disc, three tanks alive, within about 50 turns.
     overrides:
+      # HARD MAP — 128x48 arena (cordon 2 -> playable y in [2..45]).
+      # Same mid-east wall y=15..25 across x=63..78 as easy/medium.
+      # Both NORTH (y=2..7) and SOUTH (y=33..45) flank lanes open;
+      # spawn-point round-robin (NORTH staging y=12..15 / SOUTH
+      # staging y=25..28) varies the natural flank corridor per seed.
+      base_map:
+        generator: arena
+        name: combat-prevent-retreat-hard
+        width: 128
+        height: 48
+        cordon: 2
+        title: 'Prevent Retreat — Hard (Spawn-Varied)'
+        obstacles:
+          - {x: 63, y: 15, w: 16, h: 11}
       actors:
         # spawn_point 0 — NORTH staging corridor (y=12..15). Already
         # off-axis from the e1/e3 cluster (e3 Dragon range ~5, so a