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.

issues/0119-ufcs-generic-free-function-unresolved.md

@@ -32,7 +32,7 @@ Observed (one probe, all three failures):
 
 - `xs.sum_all()` (concrete fn, slice receiver) → **works**
 - `xs.first_of()` (generic `[]$T` fn, slice receiver) → `unresolved 'first_of'`
-- `p.pick(s32)` (generic `$T: Type` fn, struct receiver) → `unresolved 'pick'`
+- `p.pick(i32)` (generic `$T: Type` fn, struct receiver) → `unresolved 'pick'`
 - `a.create(Session)` (generic fn, protocol-value receiver) → `unresolved 'create'`
 
 Expected: specs.md §UFCS promises the rewrite unconditionally ("When
@@ -53,7 +53,7 @@ first_of :: (xs: []$T) -> T { xs[0] }
 
 main :: () {
     arr := .[1, 2, 3];
-    xs : []s64 = arr;
+    xs : []i64 = arr;
     print("{}\n", first_of(xs));   // 1 — direct call works
     print("{}\n", xs.first_of());  // error: unresolved 'first_of'
 }
@@ -84,7 +84,7 @@ concrete fns.
 
 Verification:
 1. The repro above prints `1` twice, exit 0.
-2. Matrix probe: generic-on-struct (`p.pick(s32)`), generic-on-slice
+2. Matrix probe: generic-on-struct (`p.pick(i32)`), generic-on-slice
    (`xs.first_of()`), generic-on-protocol-value
    (`a.create(Session)` with `create :: (a: Allocator, $T: Type) -> *T`)
    all dispatch; concrete UFCS unchanged.