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/0777-modules-bare-generic-instance-method-visible-author.sx

@@ -9,7 +9,7 @@
 // one flat hop away (visible) and `c.Box` is two hops (NOT bare-visible).
 //
 // Regression (Phase E4 finding #1, instance-method site): the static head
-// `Box(s64).make(7)` already selected `b.Box` for the layout (size 8), but the
+// `Box(i64).make(7)` already selected `b.Box` for the layout (size 8), but the
 // instance method `x.tag()` resolved by bare template name in `fn_ast_map`, so it
 // ran `c.Box.tag` (returning 7+16=23) on the b instance. Fail-before printed
 // `size=8 tag=23`; with body-author ≡ layout-author it runs b's `tag` (7+8=15).
@@ -17,8 +17,8 @@
 #import "modules/std.sx";
 #import "0777-modules-bare-generic-instance-method-visible-author/b.sx";
 
-main :: () -> s32 {
-    x := Box(s64).make(7);
+main :: () -> i32 {
+    x := Box(i64).make(7);
     print("size={} tag={}\n", size_of(type_of(x)), x.tag());
     0
 }