fix(guns): speed-sensitive caches, dead stop-shot branch, exact TM trace pairing

Four guns cached a whole prediction per tick while predict() is called once
per power bin, so every bin after the first (and the real fired shot, which
shares lastState) reused the power-1.0 lead. Fixed by caching only the
speed-INDEPENDENT derived state and recomputing the lead per requested speed:
- stop_shot: also fixes prevSpeed being written before it was read, which
  made abs(speed) < abs(prev) permanently false and the entire
  stop-prediction branch unreachable (it was just Linear).
- displacement: the cache key included bulletSpeed, so the guard missed on
  all four bins and the 15-tick window advanced ~4x/tick, making the
  inferred velocity ~4x too small.
- averaged_lead: tick cache removed outright. pattern_matcher: split into
  speed-independent match+path and per-call lead.

FeedbackEvent gains fireTick/powerBin (additive; only virtual_bullets
constructs one) so guns can pair feedback to the exact shot instead of
guessing by coordinates. tsetlin uses it: traces are now keyed exactly by
(fireTick, powerBin) with a 1024-slot ring, and the 10-frame window shifts
at most once per tick (it was shifting ~4-5x/tick, so isWarmedUp tripped
after ~2 ticks).

KNOWN INCOMPLETE: tsetlin still does not diverge from Linear in battle. The
two named bugs are fixed (a 600-tick sim shows trainedShots=2141,
traceMisses=0, and a fixed-input probe converges to a 9.6px correction), but
the TM's clause feedback itself is broken: ~131 of 1740 literals end up
included per clause, so its conjunction never fires. Sweeping TM_S,
TM_N_CLAUSES and a two-branch Type-I update did not change the correction
from 0. Needs a real TM fix or removal, not another bug fix.

First-ever guard tests for the gun selector: common_libs/tests/
test_gun_harness.nim (14 checks, headless, no Java). There were none before,
which is how six broken guns survived a full analysis cycle. Against the
previous HEAD, 5 of these checks FAIL - that is the regression guard.

common_libs/gun_harness/virtual_bullets.nim

@@ -28,6 +28,8 @@ type
     gunId*:      GunId
     powerBin*:   int       ## index into PowerBins
     targetId*:   int       ## enemy bot ID this bullet was aimed at
+    fireTick*:   int       ## tick this bullet was spawned; lets a gun pair its
+                           ## predict() trace with the exact resolution event
     fireX*, fireY*: float
     aimX*, aimY*:   float  ## predicted target (absolute)
     bulletSpeed*:  float
@@ -89,6 +91,7 @@ proc spawnBullets*(t: var VirtualTracker, gunId: GunId,
       gunId:      gunId,
       powerBin:   binIdx,
       targetId:   targetId,
+      fireTick:   state.tick,
       fireX:      state.selfX,
       fireY:      state.selfY,
       aimX:       pred.x,
@@ -144,6 +147,8 @@ proc tickBullets*(t: var VirtualTracker, state: WorldState,
       actualX:     ex,
       actualY:     ey,
       bulletPower: PowerBins[b.powerBin],
+      fireTick:    b.fireTick,
+      powerBin:    b.powerBin,
       missDistance: missDist,
       hit:         hit,
     )