Surface rename of the signed integer family: s1..s64 become i1..i64
(u1..u64, usize, isize unchanged). 'string' keeps the s-prefix arm in
name classification; width parsing moves to the i-prefix arm next to
isize.
Internal TypeId tags follow the surface (.s8/.s16/.s32/.s64 ->
.i8/.i16/.i32/.i64), as do mono-key mangle fragments (ptr_i64,
tu_i64_bool) and all display/diagnostic formatting (i{d}).
Migrated in the same sweep: stdlib + examples + issue repros + FFI C
companions (shared symbol names like ffi_id_i64), expected
stdout/stderr/ir snapshots, specs.md, readme.md, CLAUDE.md/AGENTS.md,
implementation_plan.md, docs/, issue writeups. Vendored stb_image and
historical flow state left untouched.
zig build test: 426/426; examples suite: 595/595.
lowerFor's by-value element fetch emitted index_get on the array VALUE;
the emitter realizes that as a whole-array spill to a stack temp + GEP,
per iteration — O(N^2) bytes copied per loop (and pre-0109 it also grew
the stack per iteration, segfaulting a [4096]s64 loop).
When the iterable is an array with addressable storage (and not deref'd
from a pointer, whose identifier alloca holds the pointer rather than
the array), the fetch is now index_gep on the storage + one element
load. Storage-less arrays keep the index_get fallback. The loaded
element remains a copy — mutating the capture does not write back.
Regression: examples/0048-basic-for-array-large.sx (sum over 4096
elements + by-value copy-guard).
