forth: SP@ / SP! (+4; Hayes 174/590)

plans/forth-on-sx.md

@@ -81,7 +81,7 @@ Representation:
 - [x] `S"`, `C"`, `."`, `TYPE`, `COUNT`, `CMOVE`, `FILL`, `BLANK`
 - [x] `CHAR`, `[CHAR]`, `KEY`, `ACCEPT`
 - [x] `BASE` manipulation: `DECIMAL`, `HEX`
-- [ ] `DEPTH`, `SP@`, `SP!`
+- [x] `DEPTH`, `SP@`, `SP!`
 - [ ] Drive Hayes Core pass-rate up
 
 ### Phase 5 — Core Extension + optional word sets
@@ -99,6 +99,13 @@ Representation:
 
 _Newest first._
 
+- **Phase 4 — `SP@`/`SP!` (+4; Hayes unchanged; `DEPTH` was already present).**
+  `SP@` pushes the current data-stack depth (our closest analogue to a
+  stack pointer — SX lists have no addressable backing). `SP!` pops a
+  target depth and truncates the stack via `drop` on the dstack list.
+  This preserves the save/restore idiom `SP@ … SP!` even though the
+  returned "pointer" is really a count.
+
 - **Phase 4 — `BASE`/`DECIMAL`/`HEX`/`BIN`/`OCTAL` (+9; Hayes unchanged).**
   Moved `base` from its top-level state slot into `state.vars["base"]`
   so the regular `@`/`!`/VARIABLE machinery works on it.