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.

issues/0109-loop-body-alloca-stack-growth.md

@@ -34,7 +34,7 @@ iterations; stack usage is static per frame regardless of trip count.
 
 This hits three shapes, all confirmed:
 
-1. user locals declared in a loop body (`buf : [128]s64 = ---;`),
+1. user locals declared in a loop body (`buf : [128]i64 = ---;`),
 2. nested loops (inner `for`'s `idx_slot` alloca sits in the outer body),
 3. compiler temporaries spilled in the body (e.g. `index_get`'s `ig.tmp` —
    see issue 0110 for the for-over-array case specifically).
@@ -46,10 +46,10 @@ Repro A — body local (`issues/0109-loop-body-alloca-stack-growth.sx`):
 ```sx
 #import "modules/std.sx";
 
-main :: () -> s32 {
+main :: () -> i32 {
     sum := 0;
     for 0..1000000: (i) {
-        buf : [128]s64 = ---;
+        buf : [128]i64 = ---;
         buf[0] = i;
         sum += buf[0];
     }
@@ -68,7 +68,7 @@ Repro B — pure nested loops, zero user locals:
 ```sx
 #import "modules/std.sx";
 
-main :: () -> s32 {
+main :: () -> i32 {
     n := 0;
     for 0..3000000: (i) {
         for 0..1: (j) { n += 1; }