ocean/craftax: update_mobs early-out on dead mob slots

The five move_* helpers (melee/passive/ranged mobs + mob/player
projectiles) now return immediately when mask=false. JAX's branchless
"compute-then-mask" pattern is pointless on CPU: dead slots' output
never feeds observations, rewards, or mob_map, so skipping the body
and the RNG draws is semantically equivalent.

Defining CRAFTAX_JAX_PARITY at build time restores the branchless
slow path for bitwise replay against JAX (required by
tests/craftax_parity.py). Default build uses the early-out.

Also drops craftax_step_jax_index(player_level, NUM_LEVELS) clamps at
the top of each move_* -- state->player_level is maintained in
[0, NUM_LEVELS-1] by change_floor_native (explicit bounds checks) and
by the worldgen init. Six redundant clamps per step eliminated.

Measurements (single-thread, random actions, pool=1024):
  update_mobs phase:   1.392 us -> 0.285 us (4.88x)
  full c_step:         2.35 us -> 1.22 us
  1-thread sim SPS:    425K -> 819K (1.93x)
  16-thread sim SPS:   5.53M -> 10.04M (1.82x)
  training SPS:        506K -> 544K (+7%)

Parity test with CRAFTAX_JAX_PARITY defined passes 8 seeds * 1000
steps over 27 terminals. Without the flag, parity diverges at the
first mob death -- by design.

Author: Infatoshi

ocean/craftax/step_update_mobs.h

@@ -600,13 +600,17 @@ static inline void craftax_update_mobs_move_melee(
     CraftaxThreefryKey* rng,
     int32_t index
 ) {
-    int32_t level = craftax_step_jax_index(
-        state->player_level,
-        CRAFTAX_NUM_LEVELS
-    );
+    int32_t level = state->player_level;
+    bool old_mask = state->melee_mobs.mask[level][index];
+    // Dead slot early-out: no observable effect on obs/reward/terminal.
+    // Skip body and RNG draws for speed. Breaks per-seed replay against
+    // JAX; define CRAFTAX_JAX_PARITY at build time to restore the
+    // branchless slow path (same pattern in every move_* below).
+#ifndef CRAFTAX_JAX_PARITY
+    if (!old_mask) return;
+#endif
     int32_t old_row = state->melee_mobs.position[level][index][0];
     int32_t old_col = state->melee_mobs.position[level][index][1];
-    bool old_mask = state->melee_mobs.mask[level][index];
     int32_t old_cooldown = state->melee_mobs.attack_cooldown[level][index];
     int32_t mob_type = state->melee_mobs.type_id[level][index];
 
@@ -729,13 +733,13 @@ static inline void craftax_update_mobs_move_passive(
     CraftaxThreefryKey* rng,
     int32_t index
 ) {
-    int32_t level = craftax_step_jax_index(
-        state->player_level,
-        CRAFTAX_NUM_LEVELS
-    );
+    int32_t level = state->player_level;
+    bool old_mask = state->passive_mobs.mask[level][index];
+#ifndef CRAFTAX_JAX_PARITY
+    if (!old_mask) return;
+#endif
     int32_t old_row = state->passive_mobs.position[level][index][0];
     int32_t old_col = state->passive_mobs.position[level][index][1];
-    bool old_mask = state->passive_mobs.mask[level][index];
     int32_t mob_type = state->passive_mobs.type_id[level][index];
 
     CraftaxThreefryKey draw_key =
@@ -794,13 +798,13 @@ static inline void craftax_update_mobs_move_ranged(
     CraftaxThreefryKey* rng,
     int32_t index
 ) {
-    int32_t level = craftax_step_jax_index(
-        state->player_level,
-        CRAFTAX_NUM_LEVELS
-    );
+    int32_t level = state->player_level;
+    bool old_mask = state->ranged_mobs.mask[level][index];
+#ifndef CRAFTAX_JAX_PARITY
+    if (!old_mask) return;
+#endif
     int32_t old_row = state->ranged_mobs.position[level][index][0];
     int32_t old_col = state->ranged_mobs.position[level][index][1];
-    bool old_mask = state->ranged_mobs.mask[level][index];
     int32_t old_cooldown = state->ranged_mobs.attack_cooldown[level][index];
     int32_t mob_type = state->ranged_mobs.type_id[level][index];
 
@@ -932,17 +936,17 @@ static inline void craftax_update_mobs_move_mob_projectile(
     CraftaxState* state,
     int32_t index
 ) {
-    int32_t level = craftax_step_jax_index(
-        state->player_level,
-        CRAFTAX_NUM_LEVELS
-    );
+    int32_t level = state->player_level;
+    bool old_mask = state->mob_projectiles.mask[level][index];
+#ifndef CRAFTAX_JAX_PARITY
+    if (!old_mask) return;
+#endif
     int32_t old_row = state->mob_projectiles.position[level][index][0];
     int32_t old_col = state->mob_projectiles.position[level][index][1];
     int32_t proposed_row =
         old_row + state->mob_projectile_directions[level][index][0];
     int32_t proposed_col =
         old_col + state->mob_projectile_directions[level][index][1];
-    bool old_mask = state->mob_projectiles.mask[level][index];
 
     bool proposed_in_player =
         proposed_row == state->player_position[0]
@@ -1009,17 +1013,17 @@ static inline void craftax_update_mobs_move_player_projectile(
     CraftaxState* state,
     int32_t index
 ) {
-    int32_t level = craftax_step_jax_index(
-        state->player_level,
-        CRAFTAX_NUM_LEVELS
-    );
+    int32_t level = state->player_level;
+    bool old_mask = state->player_projectiles.mask[level][index];
+#ifndef CRAFTAX_JAX_PARITY
+    if (!old_mask) return;
+#endif
     int32_t old_row = state->player_projectiles.position[level][index][0];
     int32_t old_col = state->player_projectiles.position[level][index][1];
     int32_t proposed_row =
         old_row + state->player_projectile_directions[level][index][0];
     int32_t proposed_col =
         old_col + state->player_projectile_directions[level][index][1];
-    bool old_mask = state->player_projectiles.mask[level][index];
 
     float damage_vector[3];
     craftax_update_mobs_player_projectile_damage_vector(