Extend top-level mutable `let` to cover str. The cgen now:
- emits a 16-byte zero DATAW for `let s: str;` (and the trivial
`nil` / `""` inits); a non-empty strlit init is skipped because
a compile-time .data → .text reloc isn't supported yet, so the
user gets a clean undefined-symbol error at link;
- loads `s` as `(LEAQ s(SB), CX; MOVQ (CX), AX; MOVQ 8(CX), BX)`
so the (AX=ptr, BX=len) pair convention is preserved;
- stores via the same `&s` indirection for `s = expr;` and routes
the `.ptr` / `.len` pseudo-field N_DOT branch through it; and
- tracks the declared type on each LetVar so cgident / cgdot /
cgassign pick the right load/store shape.
Selfhost cgen mirrors all four paths byte-for-byte; test 990
(cgen-match on err.ww) and tests 994/995 (self-rebuild) stay
green. 630_let_global gains two new fixtures (`let msg: str;` +
runtime assign, plus reassign from a helper).
Slice and struct globals still NYI — same scope deferred.
Hojun-Cho
208bdd25df