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/expected/0517-packs-pack-reflection-intrinsics.stdout

@@ -1,8 +1,8 @@
-s64 string bool
+i64 string bool
 true false true false
 inline-if folded: same
 Allocator/CAllocator: true
-Allocator/s64: false
-Wrap(s64)/s32: true
-Wrap(s64)/bool: false
-Wrap(bool)/s32: false
+Allocator/i64: false
+Wrap(i64)/i32: true
+Wrap(i64)/bool: false
+Wrap(bool)/i32: false