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/0735-modules-flat-same-name-fn-value-winner.sx

@@ -1,7 +1,7 @@
 // fix-0102d site 2 / attempt-2 (issue 0102): the first-wins winner's body is
 // independently BROKEN (references an undefined symbol) and is never used. A
 // shadow author from a later flat import takes its OWN `pick` as a function
-// VALUE (`g : () -> s64 = pick`). The value must bind the shadow (own-author
+// VALUE (`g : () -> i64 = pick`). The value must bind the shadow (own-author
 // wins) and the broken winner must NOT be lowered — a rerouted fn value never
 // uses the winner. Before the fix the fn-value site eagerly lazily-lowered the
 // name-keyed winner BEFORE the resolver rerouted, surfacing the winner's
@@ -11,7 +11,7 @@
 #import "0735-modules-flat-same-name-fn-value-winner/a.sx";
 #import "0735-modules-flat-same-name-fn-value-winner/b.sx";
 
-main :: () -> s32 {
+main :: () -> i32 {
     print("from_b_value = {}\n", from_b_value());
     0
 }