Three-trigger keyboard weapon groups: 1=lasers 2=PPCs 3=missiles (task #43)

Interim pod-like grouping ahead of the authentic ConfigureMappables/
ChooseButton mapper channels (decomp-anchored in open-questions):
- Three fire channels like three pod buttons: gBTWeaponTrigger (lasers,
  key 1 or SPACE), gBTPPCTrigger (PPCs, key 2), gBTMissileTrigger
  (missiles, key 3 or CTRL). Keyboard only per user (mouse buttons
  removed).
- EmitterSimulation picks its channel by classID (PPC 3028 vs Emitter
  3016); BT_AUTOFIRE holds all three.
- The BT_BEAM_LOG sampler modulus 30 aliased to ONE weapon under the
  new synchronized 5-beam volleys (5 | 30) -> 31 (coprime).

Verified: all 5 energy weapons fire (31 samples each), aimed hits land,
kill completes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

context/build-and-run.md

@@ -39,7 +39,8 @@ run\run.cmd [EGG]      # default DEV.EGG; cd's to content\ and runs btl4.exe -eg
 - **cwd MUST be `content\`** — the engine resolves `BTL4.RES`, `VIDEO\`, `BTDPL.INI`, eggs relative
   to cwd (the `loadTables` gotcha: `L4VIDEO.cpp:849` `fopen("VIDEO\\REPLACEMATS.tbl")` is relative +
   unchecked → fread on NULL if cwd is wrong). Logs to `btl4.log` in `content\` (or `BT_LOG=<file>`). [T2]
-- Interactive: **WASD** drive, **Space/Ctrl** fire, **X** all-stop. Default egg = `DEV.EGG`
+- Interactive: **WASD** drive; weapon groups (keyboard, task #43) **1/Space** = lasers, **2** =
+  PPCs, **3/Ctrl** = missiles; **X** all-stop; **V** cockpit/chase view. Default egg = `DEV.EGG`
   (map=grass, time=day). Swap mech via the egg's `vehicle=` (bhk1/madcat/…). The code path is
   mech-agnostic (verified — Mad Cat booted + fought with zero code changes). [T2]