Replace 13 Rust host functions with Forth bootstrap (Phase 1)

Create boot.fth loaded at startup after IR primitives are compiled.
Forth-compiled WASM with direct calls outperforms host function dispatch
(no call_indirect overhead, Cranelift can inline across word boundaries).

Words moved to Forth: 2OVER, 2ROT, WITHIN, 2@, 2!, FILL, CMOVE, CMOVE>,
MOVE, ERASE, BLANK, /STRING, -TRAILING.

Removed 547 lines of Rust closures, replaced by 48 lines of Forth.
All 425 tests pass.

crates/core/boot.fth

@@ -0,0 +1,61 @@
+\ WAFER Bootstrap -- Forth definitions replacing Rust host functions.
+\ Loaded at startup after IR primitives are compiled.
+\ Compiled WASM with direct calls outperforms host function dispatch.
+
+\ ---------------------------------------------------------------
+\ Phase 1: Pure stack and memory operations
+\ ---------------------------------------------------------------
+
+\ 2OVER ( x1 x2 x3 x4 -- x1 x2 x3 x4 x1 x2 )
+: 2OVER  3 PICK 3 PICK ;
+
+\ 2ROT ( x1 x2 x3 x4 x5 x6 -- x3 x4 x5 x6 x1 x2 )
+: 2ROT   2>R 2SWAP 2R> 2SWAP ;
+
+\ WITHIN ( n lo hi -- flag )  true if lo <= n < hi (unsigned)
+: WITHIN  OVER - >R - R> U< ;
+
+\ 2@ ( addr -- x1 x2 )  fetch double-cell, low addr = deeper stack
+: 2@  DUP CELL+ @ SWAP @ ;
+
+\ 2! ( x1 x2 addr -- )  store double-cell
+: 2!  SWAP OVER ! CELL+ ! ;
+
+\ 2R@ stays as host function (needs direct return-stack access)
+
+\ FILL ( addr u char -- )  fill u bytes with char
+: FILL  ROT ROT  0 ?DO 2DUP I + C! LOOP 2DROP ;
+
+\ CMOVE ( src dst u -- )  forward byte copy
+: CMOVE  0 ?DO OVER I + C@ OVER I + C! LOOP 2DROP ;
+
+\ CMOVE> ( src dst u -- )  backward byte copy (for overlap)
+: CMOVE>
+  DUP 0= IF DROP 2DROP EXIT THEN
+  1- >R
+  BEGIN R@ 0< INVERT WHILE
+    OVER R@ + C@  OVER R@ + C!
+    R> 1- >R
+  REPEAT
+  R> DROP 2DROP ;
+
+\ MOVE ( src dst u -- )  smart copy (handles overlap)
+: MOVE
+  DUP 0= IF DROP 2DROP EXIT THEN
+  >R 2DUP U< IF R> CMOVE> ELSE R> CMOVE THEN ;
+
+\ ERASE ( addr u -- )  zero fill
+: ERASE  0 FILL ;
+
+\ BLANK ( addr u -- )  space fill
+: BLANK  BL FILL ;
+
+\ /STRING ( addr u n -- addr+n u-n )
+: /STRING  ROT OVER + ROT ROT - ;
+
+\ -TRAILING ( addr u -- addr u' )  remove trailing spaces
+: -TRAILING
+  BEGIN DUP 0> WHILE
+    2DUP + 1- C@ BL <> IF EXIT THEN
+    1-
+  REPEAT ;