j147: the 1-tick fire-detection lag is OURS — measure it, then back-date it (TR_FIRE_LAG)

MEASURED LIVE (common_libs/tests/measure_fire_ghost_lag.py, 4 sessions, 1777
matched ghost spawns, both movers): the server dispatches a turn's fire AFTER
our go() for that same turn, so a turn-T shot's energy drop first reaches our
scan at turn T+1 — and a bullet takes its FIRST step during the turn it is
fired, so the true bullet is already one whole bullet step (11-20 px) downrange.
Both movers place the ghost at the SCANNED enemy position (where the bullet was
born), so the whole ghost trajectory is the true one shifted one turn later and
the arrival deadline is a full tick late.

MEASURED: detection lag +1 tick on 100% of 1777 matched spawns; ghost-vs-
observer displacement 19.06 px mean / 22.00 p90 (tfil) and 16.08 / 21.81
(strafe); arrival-deadline error 0.99 / 0.77 ticks. NOT a rendering artefact:
the draw/advance order is correct (advanceBullets -> detectFires -> build).

THE FIX: TR_FIRE_LAG (int, default 0 = today byte-for-byte) in the shared
fire_tracker, applied by both movers at spawn: x = origin + dir*speed*lag.
The deadline needs no separate change — both movers derive it from the ghost's
own position, so a correct position gives a correct deadline.
WITH IT: displacement 19.06 -> 5.37 px mean (the residue is the enemy's own
<=8 px scan staleness) and the deadline error 0.99 -> 0.06 ticks.

Guards: test_tfil_commit_env 77 -> 87 checks (default golden parity, exact
n-step back-date, deadline shortens by exactly lag, junk/negative degrade to 0,
reaped exactly one tick earlier); test_env_report + test_env_dotenv green.
TR_FIRE_LAG registered in env_report + knownEnvNames + .env.example +
docs/env_reference.md. Live A/B pre-registered in docs/movement_campaign.md
(Batch 8) with its MDE stated up front; arms tools/ab/arms_fire_lag.txt.
TR_FIRE_DIAG gains a per-round ROUND line (the tick->getTurn anchor) and a
per-spawn SPAWN line (the ghost's drawn position).

common_libs/movement_harness/fire_tracker.nim

@@ -134,6 +134,38 @@ proc detect*(t: var FireTracker, id: int, energy: float,
   elif drop >= lo and drop <= hi:
     result = @[drop]
 
+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
+## 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
+## the ghost at the SCANNED enemy position, i.e. exactly where the bullet was
+## born: the whole ghost trajectory is the true one shifted one turn later.
+## The per-tick `advanceBullets` then keeps it there for the bullet's whole life.
+##
+## The compensation is the inverse: at SPAWN, back-date the shot by `lag` ticks
+## (`x = origin + dir * speed * lag`, `y = ...`).  The arrival deadline needs no
+## separate change — every mover derives it from the ghost's own position
+## (`heatDecay(along / speed)`, the `dot < 0` reap), so a correct position gives a
+## correct deadline.
+##
+## DEFAULT 0 = the shipped behaviour, byte for byte (`x` is only touched when
+## `lag > 0`), so the default-parity guard stays green.
+var FireLag*: int = 0
+
+proc loadFireTrackerEnv*() =
+  ## Read the shared fire knobs.  Called once at module init; callable again
+  ## after `putEnv` so a guard test can exercise the arms in one process.
+  let s = getEnv("TR_FIRE_LAG", "").strip()
+  FireLag = (try: max(0, parseInt(s)) except ValueError: 0)
+
+loadFireTrackerEnv()
+
 proc endScan*(t: var FireTracker) =
   ## Call once after the per-enemy scan.  Rotates the event corrections one
   ## slot: the events noted since the previous `endScan` become the corrections