forth: IF/ELSE/THEN + PC-driven body runner (+18)

plans/forth-on-sx.md

@@ -69,7 +69,7 @@ Representation:
 - [x] Tests in `lib/forth/tests/test-phase2.sx` — 26/26 pass
 
 ### Phase 3 — control flow + first Hayes tests green
-- [ ] `IF`, `ELSE`, `THEN` — compile to SX `if`
+- [x] `IF`, `ELSE`, `THEN` — compile to SX `if`
 - [ ] `BEGIN`, `UNTIL`, `WHILE`, `REPEAT`, `AGAIN` — compile to loops
 - [ ] `DO`, `LOOP`, `+LOOP`, `I`, `J`, `LEAVE` — counted loops (needs a return stack)
 - [ ] Return stack: `>R`, `R>`, `R@`, `2>R`, `2R>`, `2R@`
@@ -99,6 +99,19 @@ Representation:
 
 _Newest first._
 
+- **Phase 3 start — `IF`/`ELSE`/`THEN` (+18).** `lib/forth/compiler.sx`
+  + `tests/test-phase3.sx`. Colon-def body switched from `for-each` to
+  a PC-driven runner so branch ops can jump: ops now include dict tags
+  `{"kind" "bif"|"branch" "target" cell}` alongside the existing
+  `(fn (s) ...)` shape. IF compiles a `bif` with a fresh target cell
+  pushed to `state.cstack`; ELSE emits an unconditional `branch`,
+  patches the IF's target to the instruction after this branch, and
+  pushes the new target; THEN patches the most recent target to the
+  current body length. Nested IF/ELSE/THEN works via the cstack.
+  Also fixed `EMIT`: `code-char` → `char-from-code` (spec-correct
+  primitive name) so Phase 1/2 tests run green on sx_server.
+  152/152 (Phase 1 + 2 + 3) green.
+
 - **Phase 2 complete — colon defs, compile mode, VARIABLE/CONSTANT/VALUE/TO, @/!/+! (+26).**
   `lib/forth/compiler.sx` plus `tests/test-phase2.sx`.
   Colon-def body is a list of ops (one per source token) wrapped in a single