feat: implement Rust backends for MUST panels — Valence Shell, Game Preview, VM Inspector

Add Tauri command implementations for three critical IDApTIK panels:
- Valence Shell: 12 commands (PTY spawn, asciicast recordings, checkpoint
  create/restore, approval gate, screenshot) with 16 tests
- Game Preview: 8 commands (dev server health, game loop control, clip
  recording, render stats, screenshot) with 9 tests
- VM Inspector: 7 commands (step forward/backward, run, load program,
  export snapshot) with full instruction execution for all 23 mnemonics
  (ADD, SUB, MUL, DIV, PUSH, POP, LOAD, STORE, SWAP, XOR, AND, OR,
  FLIP, ROL, ROR, NEGATE, NOOP, IF_ZERO, IF_POS, LOOP, CALL, SEND,
  RECV), 256-cell memory, 10k-snapshot history ring buffer, 16 tests.

Total: 27 new Tauri commands, 41 new tests (72 Rust tests total).
Note: Rust tests require --test-threads=1 for stateful modules.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: hyperpolymath

src-tauri/Cargo.toml

@@ -36,6 +36,10 @@ notify-debouncer-mini = "0.5"
 walkdir = "2"
 regex = "1"
 libc = "0.2"
+which = "7"
+
+[dev-dependencies]
+tokio = { version = "1", features = ["macros", "rt"] }
 
 [features]
 default = ["custom-protocol"]