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/0749-modules-namespaced-only-bare-param-alias-not-visible.sx

@@ -1,6 +1,6 @@
 // Bare PARAMETERIZED-struct alias visibility under a NAMESPACED-only import —
 // the generic-struct sibling of 0747 (plain alias) and 0743 (named type). A
-// generic-struct instantiation alias (`Secret :: Box(s32)`) registers ONLY a
+// generic-struct instantiation alias (`Secret :: Box(i32)`) registers ONLY a
 // named struct type in the TypeTable; its raw import fact stays `.const_decl`,
 // so before the fix it was NOT recognised as a type author and a BARE `Secret`
 // leaked to the registered struct with NO diagnostic (the value silently came
@@ -11,7 +11,7 @@
 // (attempt-5 R4-parameterized-alias-leak).
 dep :: #import "0749-modules-namespaced-only-bare-param-alias-not-visible/dep.sx";
 
-main :: () -> s32 {
+main :: () -> i32 {
     s : Secret = .{ value = 42 };
     s.value
 }