fix(runtime): pin MC_VERSION=1.21.4 + ground-truth probe + close-loop dig

Two-pronged response to user-confirmed "bot stands still, doesn't actually
chop" on play.xmatic.team:

1. Pin protocol — .env now sets MC_VERSION=1.21.4. minecraft-data has
   wrong packet ID mappings for protocol 775 (server 26.1.2 via
   ViaBackwards 5.9.1) — see mineflayer#3888 and #3717. 1.21.5 also has
   an enchants decoder bug that breaks bot.dig. 1.21.4 is the last
   protocol mineflayer 4.37.1 can speak cleanly through VIA.

2. Don't trust dig success — runtime/actions.js chopNearestTree and
   runtime/skills/gather-stone.js now lookAt(face center)+forceLook,
   await collectBlock, then re-read the target block. If the log/stone
   is STILL there, return ok:false code:"silent_dig_failure" and
   blacklist the position. Prevents the curriculum from reporting
   "wood.16 in progress" while the world hasn't actually changed.

3. Defensive default — runtime/movement-profiles.js: canDig=false on
   every profile until dig is confirmed working live. Otherwise
   pathfinder schedules paths through must-dig blocks and the bot loops.

4. Ground-truth probe — runtime/skills/diagnose-physics.js dispatches
   forward/jump/dig probes and writes the result to the diary. New
   IPC command cmd:run-skill lets the operator (or a future curriculum
   trigger) fire any skill on demand; it waits for the current action
   to finish before dispatching. /tmp/pepa-runskill.mjs is a one-shot
   client.

Live probe on play.xmatic.team confirmed: forward Δ=0.003 over 2s
(BROKEN — server rejects movement packets), jump ΔY=0.42 (likely
physics jitter, not a real jump). Strongly suggests an anti-cheat
plugin gating bot-style movements server-side — beyond protocol pin.

npm test 124/124.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

runtime/ipc-protocol.js

@@ -27,6 +27,7 @@ export const COMMAND_TYPES = Object.freeze({
 	SNAPSHOT: "cmd:snapshot", // request immediate STATUS event
 	PROPOSAL_LATEST: "cmd:proposal-latest", // request latest pending proposal
 	PROPOSAL_APPROVE: "cmd:proposal-approve", // { filename } move to approved/
+	RUN_SKILL: "cmd:run-skill", // { skillId, args? } dispatch a skill once (operator ground-truth probes)
 });
 
 export function encodeFrame(obj) {