feat(codegen): stack under/overflow guards in compiled words (WS-007)

Compiled code could silently move dsp/rsp/fsp out of their stack
regions (e.g. DROP on an empty stack), corrupting later pushes with
no diagnostic -- the addresses stay inside valid linear memory, so
nothing could trap. Host-side checks cannot catch it.

- Guards are emitted at the sp-adjustment choke points (dsp_inc/
  dsp_dec, fsp_inc/fsp_dec, rpush/rpop/rpeek, peek, TwoDup/TwoDrop,
  promoted prologue/epilogue -- DROP never loads its value, so
  guarding pop() alone is not enough). On fault: write the code to
  SYSVAR_FAULT_CODE, call _STACK_FAULT_, which THROWs it -- so
  guards are CATCHable and print standard messages (-3/-4/-5/-6/
  -44/-45).
- The batch/consolidated compile path (all boot primitives) and the
  export path are wired too; a thread-local carries the fault index
  into the shared emission helpers.
- Config: codegen.stack_guards, default ON. `wafer build` output
  defaults OFF (production artifact); WAFER_STACK_GUARDS=0|1
  overrides either. Perf comparison lanes run unguarded.
- Measured overhead in release loops: within noise (never-taken
  branches).
- toolstest.fth baseline 37 -> 38: line 368's bare interpreted `R>`
  used to underflow silently and count as passing; the guard now
  correctly reports -6.

crates/core/tests/comparison.rs

@@ -93,6 +93,8 @@ fn find_sf64() -> Option<&'static str> {
 /// Spawn `binary`, write `input` to its stdin, and collect the output.
 fn run_via_stdin(binary: &str, input: &str) -> Option<std::process::Output> {
     Command::new(binary)
+        // Perf lanes measure unguarded code (only the wafer binary reads this)
+        .env("WAFER_STACK_GUARDS", "0")
         .stdin(std::process::Stdio::piped())
         .stdout(std::process::Stdio::piped())
         .stderr(std::process::Stdio::piped())