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.

src/token.zig

@@ -227,7 +227,7 @@ pub const Token = struct {
     tag: Tag,
     loc: Loc,
     /// True when an `.identifier` was introduced by a leading backtick
-    /// (`` `s2 ``): a RAW identifier whose text excludes the backtick and which
+    /// (`` `i2 ``): a RAW identifier whose text excludes the backtick and which
     /// the parser must NEVER type-classify (it bypasses the reserved-type-name
     /// rule). `loc` already spans only the un-backticked name, so `slice` returns
     /// the bare text.