`S name` in interpret mode used to leave (c-addr u) pointing into the
input buffer, so the next REFILL clobbered the bytes. Typing `s test`
then `type` on a fresh line printed "pest" because the new input
overwrote the first chars of the old TIB content.
Move `S` from boot.fth to the Rust outer interpreter alongside `S"` /
`C"`: both interpret and compile modes now copy the token to HERE-space
(stable across REFILL). Compile-mode output is still bit-identical to
writing `S" name"` inline.
Adds `test_s_interpret_survives_refill` regression.
ok2
d1a7d55051