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/0209-generics-value-param-integral-float.sx

@@ -1,8 +1,8 @@
-// A generic value parameter (`$K: u32`) binds a literal (`Vec(3, s64)`) and an
-// integral-float named const (`Vec(L, s64)` with `L : f64 : 4.0`) to the same
+// A generic value parameter (`$K: u32`) binds a literal (`Vec(3, i64)`) and an
+// integral-float named const (`Vec(L, i64)` with `L : f64 : 4.0`) to the same
 // integer a plain `4` would — the value-param arg folds through the shared
 // const-int evaluator, so the integral-float rule (F0.4 attempt 8, Agra ruling)
-// reaches value params too. The folded value is the array length `[K]s64`.
+// reaches value params too. The folded value is the array length `[K]i64`.
 //
 // The bind is range-checked against the declared `u32` (an out-of-range arg is a
 // clean compile error — see 1134); a valid in-range value binds normally.
@@ -13,7 +13,7 @@ Vec :: struct ($K: u32, $T: Type) { data: [K]T; }
 L : f64 : 4.0;
 
 main :: () {
-    a : Vec(3, s64) = ---;       // literal value param
-    b : Vec(L, s64) = ---;       // integral-float named-const value param → 4
+    a : Vec(3, i64) = ---;       // literal value param
+    b : Vec(L, i64) = ---;       // integral-float named-const value param → 4
     print("a.len={} b.len={}\n", a.data.len, b.data.len);   // 3 and 4
 }