lang: rename signed integer types sN -> iN

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.

examples/0741-modules-flat-same-name-bare-pack-winner/a.sx

@@ -2,5 +2,5 @@
 // a.sx authors `f` as a comptime pack; imported first → first-wins winner.
 // `show_a`'s bare `f()` is the caller's OWN author (own == winner → existing
 // pack path, byte-for-byte unchanged): dispatched as a.f (the pack → 1).
-f :: (..$args) -> s64 { return 1; }
+f :: (..$args) -> i64 { return 1; }
 show_a :: () { print("a: f() = {}\n", f()); }

examples/0741-modules-flat-same-name-bare-pack-winner/b.sx

@@ -2,5 +2,5 @@
 // b.sx authors its OWN plain `f`. `show_b`'s bare `f()` must dispatch b.f (2),
 // not the first-wins pack winner from a.sx (1). The selector picks b.f; the
 // early pack/comptime dispatch must NOT hijack it with the winner.
-f :: () -> s64 { return 2; }
+f :: () -> i64 { return 2; }
 show_b :: () { print("b: f() = {}\n", f()); }