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/1145-diagnostics-missing-struct-field-assign.sx

@@ -4,25 +4,25 @@
 // into a neighbouring field.
 //
 // Regression (issue 0094): the lvalue field lookup left `field_ty = .unresolved`
-// (lowerAssignment's assignment-target path) or silently GEP'd field 0 as `.s64`
+// (lowerAssignment's assignment-target path) or silently GEP'd field 0 as `.i64`
 // (lowerExprAsPtr's fallback / lowerMultiAssign's struct loop), so a missing-field
 // store either built a pointer-to-`.unresolved` that panicked at LLVM emission or
 // silently wrote field 0. All three lvalue sites now emit the field-not-found
 // diagnostic: the assignment-target path (`p.q`), the nested lvalue-pointer path
 // (`o.missing.a`), and the multi-target store path (`p.r, y`).
 
-Point :: struct { x: s64; }
-Inner :: struct { a: s64; }
+Point :: struct { x: i64; }
+Inner :: struct { a: i64; }
 Outer :: struct { inner: Inner; }
 
-main :: () -> s32 {
+main :: () -> i32 {
     p := Point.{ x = 1 };
     p.q = 2;                              // site 1: lowerAssignment target path
 
     o := Outer.{ inner = Inner.{ a = 1 } };
     o.missing.a = 5;                      // site 2: lowerExprAsPtr fallback
 
-    y : s64 = 0;
+    y : i64 = 0;
     p.r, y = 3, 4;                        // site 3: lowerMultiAssign field path
 
     return 0;