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.
A value-position match's arms are now lowered with `target_type` set to
the merge's `result_type`, so positive and negated integer literals pick
the same width. Fixes the `PHI node operands are not the same type as the
result` failure for `if n == { case 0: 100; else: -1; }`-style returns.
Regression: examples/0043-basic-match-value-mixed-width.sx.
Gates: zig build, zig build test, run_examples.sh -> 345 passed.
