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/0709-modules-issue-0056/common.sx

@@ -3,10 +3,10 @@
 // Impl blocks are anonymous (`declName() == null`), so the diamond-import
 // dedup in imports.zig (`mergeFlat`) skips them and appends the node once per
 // path — `registerParamImpl` then trips its same-file duplicate check.
-Wrapped :: struct { v: s64; }
+Wrapped :: struct { v: i64; }
 
-impl Into(Wrapped) for s64 {
-    convert :: (self: s64) -> Wrapped {
+impl Into(Wrapped) for i64 {
+    convert :: (self: i64) -> Wrapped {
         return .{ v = self };
     }
 }

examples/0709-modules-issue-0056/mid_a.sx

@@ -1,3 +1,3 @@
 #import "common.sx";
 
-mid_a_marker :: () -> s64 { 1 }
+mid_a_marker :: () -> i64 { 1 }

examples/0709-modules-issue-0056/mid_b.sx

@@ -1,3 +1,3 @@
 #import "common.sx";
 
-mid_b_marker :: () -> s64 { 2 }
+mid_b_marker :: () -> i64 { 2 }