FIX an incomplete commit: TMHORIZON was admitted unconditionally at HEAD

MY ERROR. `aed579b` committed `ModularBot.nim` (which wires the new gun as id 15)
and `guns/tm_horizon.nim`, but I staged only three files and left job-72's RACK
REGISTRATION behind in the working tree. Consequence at HEAD:

- `common_libs/gun_harness/selector.nim` still had the 15-entry rack table
  (ids 0..14) with no TMHORIZON name, so
- `TR_RACK_TMHORIZON` was never read (a silent no-op), and
- id 15 is PAST the table, and `rackAdmitted` documents "an id past the
  membership table is admitted" -> **TMHORIZON was admitted unconditionally**.

So the committed default rack was `Pattern + TMHORIZON`, not the intended
`Pattern only`, and the gun could not be switched off by env at HEAD. This was
caught by a live A/B job that had to work around it with `GUN_RACK_DISABLE=15`.

FIX (this commit): the rack table goes to 16 entries with `TMHORIZON` appended at
id 15 defaulting to `rmOff`, plus the two tests whose length literals follow it
(`test_rack_membership` 15->16, `test_tm_pattern_registration` 15->16).
`DefaultRackMembership` is again Pattern-only with every other gun `off`.

NOTE ON SCOPE: `selector.nim` in the working tree also contains a CONCURRENT job's
power-policy threading (an `enemyEnergy` parameter). That work is still in flight
and is deliberately NOT included here - only the three rack hunks were staged.
The remainder stays unstaged for its own commit.

The lesson, recorded because it has bitten twice tonight in different forms: a
change is not committed until its registration/table counterpart is, and
`git add` of a hand-picked file list is exactly how a half-change ships.

common_libs/gun_harness/selector.nim

@@ -44,10 +44,10 @@ import virtual_bullets
 # transition the radar uses.
 
 const
-  RackGunNames*: array[15, string] = [
+  RackGunNames*: array[16, string] = [
     "HEADON", "LINEAR", "TSETLIN", "CIRCULAR", "GUESSFACTOR", "PATTERN",
     "WALLBOUNCE", "ACCEL", "STOPSHOT", "DISPLACE", "AVGLEAD", "DECAYGF",
-    "KNN", "TMSELECT", "TMPATTERN"]
+    "KNN", "TMSELECT", "TMPATTERN", "TMHORIZON"]
   RackEnvPrefix* = "TR_RACK_"
   ## SHIPPED DEFAULT: `onlyPattern`. Pattern (id 5) is admitted in both racks;
   ## every other gun is `off`. The selection mechanism is untouched and remains
@@ -57,7 +57,7 @@ const
   ## it never spawns a virtual bullet unless explicitly enabled, so the shared
   ## VirtualTracker ring head — and every other gun's learning order — is
   ## unchanged.
-  DefaultRackMembership*: array[15, RackMembership] = [
+  DefaultRackMembership*: array[16, RackMembership] = [
     rmOff,  # 0  HEADON      — off (measured: worst over-selected gun)
     rmOff,  # 1  LINEAR      — off
     rmOff,  # 2  TSETLIN     — off
@@ -72,7 +72,8 @@ const
     rmOff,  # 11 DECAYGF     — off
     rmOff,  # 12 KNN         — off
     rmOff,  # 13 TMSELECT    — off
-    rmOff]  # 14 TMPATTERN   — off (already shipped off; TM pattern gun)
+    rmOff,  # 14 TMPATTERN   — off (already shipped off; TM pattern gun)
+    rmOff]  # 15 TMHORIZON   — off (horizon-based TM corrector; expected to lose)
 
 proc parseRackMembership*(value: string): RackMembership =
   ## Parse a `TR_RACK_<GUN>` value. Empty / unknown values fall back to the