Sync HERE to WASM memory, replace HERE host function with Forth (Phase 4)

HERE is now defined in boot.fth as `: HERE 12 @ ;` (reads SYSVAR_HERE
from WASM linear memory). The Rust side syncs user_here to memory[12]:
- At the start of each evaluate() call (sync_here_to_wasm)
- In each host function that modifies HERE (ALLOT, comma, C-comma, ALIGN)

This avoids per-token sync overhead — only 2 sync points per evaluate()
call plus host-function writes. Removed the HERE host function closure
(~30 lines). All 426 tests pass.

crates/core/boot.fth

@@ -137,3 +137,14 @@
 
 \ */MOD ( n1 n2 n3 -- rem quot )
 : */MOD  >R M* R> FM/MOD ;
+
+\ ---------------------------------------------------------------
+\ Phase 4: HERE and ALIGNED
+\ ---------------------------------------------------------------
+
+\ HERE reads from SYSVAR_HERE (offset 12 in WASM memory).
+\ The Rust side syncs user_here to memory[12] before each evaluate call
+\ and whenever host ALLOT/comma modifies it.
+: HERE  12 @ ;
+
+\ ALIGNED is already an IR primitive in the compiler.