j147 results: 180-battle live A/B on the frozen 15-opponent panel

Within-mover references (the only comparison that isolates the knob): tfil_lag1
-0.04 wins/run, strafe_lag1 +0.18 wins/run, both far under the reported MDEs
(0.33 / 0.41) -> NOT distinguishable, so TR_FIRE_LAG stays default 0. Incoming
hit rate also unmoved (+0.94 / +0.33 pp). The one significant result is the
mover (strafe_lag1 vs tfil_off +0.38 wins/run p=0.0386, hit rate -4.67pp
p=0.0074), which is the known strafe-over-tfil gap and exactly why the
within-mover reference was pre-registered.

Also a cosmetic no-op refactor of the two spawn sites (compute velX/velY once;
bit-identical, and it keeps the default-parity claim exact).

common_libs/movement_harness/fire_tracker.nim

@@ -137,10 +137,10 @@ proc detect*(t: var FireTracker, id: int, energy: float,
 import std/[math, os, strutils]
 
 ## ── j147: the DETECTION LAG back-date (`TR_FIRE_LAG`, default 0) ─────────────
-## MEASURED LIVE (`common_libs/tests/measure_fire_ghost_lag.py`, 1171/1171 ghost
-## spawns over two movers x 4 rounds, `TR_FIRE_DIAG=1`): the server dispatches a
-## turn's fire AFTER our `go()` for that same turn, so the energy drop of a
-## turn-T shot first reaches our scan at turn T+1 (our bot tick T).  A bullet
+## MEASURED LIVE (`common_libs/tests/measure_fire_ghost_lag.py`, 4 sessions,
+## 1777 matched ghost spawns over both movers, `TR_FIRE_DIAG=1`): the server
+## dispatches a turn's fire AFTER our `go()` for that same turn, so the energy
+## drop of a turn-T shot first reaches our scan at turn T+1 (our bot tick T).  A bullet
 ## takes its FIRST step during the turn it is fired, so by then the true bullet
 ## is already `speed` px (11..20 px, one whole bullet step) downrange and the
 ## arrival deadline is a full tick shorter than the ghost's.  Both movers place

common_libs/movements/strafe.nim

@@ -579,16 +579,17 @@ proc spawnTrackedWave(m: var StrafeModule, ws: WorldState, ei: EnemyInfo,
     m.bullets.del(0)
   # j147: back-date the shot by the measured detection lag (`TR_FIRE_LAG`,
   # default 0 = untouched).  See `movement_harness/fire_tracker.nim`.
+  let vx = speed * cos(heading)
+  let vy = speed * sin(heading)
   var gx = ei.x
   var gy = ei.y
   if FireLag > 0:
-    gx += (speed * cos(heading)) * FireLag.float
-    gy += (speed * sin(heading)) * FireLag.float
+    gx += vx * FireLag.float
+    gy += vy * FireLag.float
   m.bullets.add TrackedBullet(
     originX: ei.x, originY: ei.y,
     x: gx, y: gy,
-    velX: speed * cos(heading),
-    velY: speed * sin(heading),
+    velX: vx, velY: vy,
     power: power, alive: true, age: 0)
 
 proc noteEnemyBulletHit*(m: var StrafeModule, power: float) =

common_libs/movements/the_floor_is_lava.nim

@@ -433,16 +433,18 @@ proc spawnTrackedWave(m: var TFILModule, ws: WorldState, ei: EnemyInfo,
     m.bullets.del(0)  # ponytail: drop oldest; fine for 20-bullet cap
   # j147: back-date the shot by the measured detection lag (`TR_FIRE_LAG`,
   # default 0 = untouched).  See `movement_harness/fire_tracker.nim`.
+  let vx = speed * cos(heading)
+  let vy = speed * sin(heading)
   var gx = ei.x
   var gy = ei.y
   if FireLag > 0:
-    gx += (speed * cos(heading)) * FireLag.float
-    gy += (speed * sin(heading)) * FireLag.float
+    gx += vx * FireLag.float
+    gy += vy * FireLag.float
   m.bullets.add TrackedBullet(
     originX: ei.x, originY: ei.y,
     x: gx, y: gy,
-    velX: speed * cos(heading),
-    velY: speed * sin(heading),
+    velX: vx,
+    velY: vy,
     power: power,
     alive: true,
     age: 0)