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/ir/lower/comptime.zig

@@ -287,7 +287,7 @@ pub fn evalComptimeInt(self: *Lowering, node: *const Node) ?i64 {
 pub fn lowerComptimeGlobal(self: *Lowering, name: []const u8, expr: *const Node, type_ann: ?*const Node) void {
     // When the user writes `NAME :: #run expr;` with no type annotation,
     // infer the global's type from the comptime expression's return
-    // shape. `resolveType(null)` returns `.s64` for legacy reasons —
+    // shape. `resolveType(null)` returns `.i64` for legacy reasons —
     // good for primitive helpers, silently wrong for anything else.
     const expr_ty = self.inferExprType(expr);
     // A failable `#run` (bare, no `catch`/`or`): the comptime function
@@ -1130,7 +1130,7 @@ pub fn foldComptimeFloatInit(self: *Lowering, node: *const Node, dst: TypeId) ?R
     // `evalConstFloatExpr` only succeeds for literal / const-arithmetic
     // nodes, never an unbound pack index. `inferExprType` is the primary
     // signal, but it reads a const's DECLARED type — which is a placeholder
-    // `s64` for an untyped float-EXPRESSION const (`ME :: 4.0 + 1.0`), so
+    // `i64` for an untyped float-EXPRESSION const (`ME :: 4.0 + 1.0`), so
     // `ME / 2` would look like integer division; `isFloatValuedExpr` (judging
     // by VALUE) catches that case so it narrows under the unified rule too.
     if (!isFloat(self.inferExprType(node)) and !program_index_mod.isFloatValuedExpr(node, self)) return null;