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/1124-diagnostics-imported-reserved-destructure.sx

@@ -1,4 +1,4 @@
-// A reserved type name used as a DESTRUCTURE binding name (`s2`) is rejected
+// A reserved type name used as a DESTRUCTURE binding name (`i2`) is rejected
 // even when it lives in an IMPORTED module — the exhaustive binding-name walk
 // descends the `namespace_decl` an `mod :: #import` wraps and renders the
 // diagnostic against that module's source (issue 0077's universal-coverage
@@ -6,10 +6,10 @@
 // lowering and aborts LLVM verification.
 //
 // Regression (issues 0076 + 0077, attempt-4 coverage). Expected: one clean
-// diagnostic pointing at the imported module's `s2, rest := ...`, exit 1.
+// diagnostic pointing at the imported module's `i2, rest := ...`, exit 1.
 #import "modules/std.sx";
 mod :: #import "1124-diagnostics-imported-reserved-destructure/mod.sx";
 
-main :: () -> s32 {
+main :: () -> i32 {
     return mod.run();
 }