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/0115-same-name-const-scalar-array-collision.md

@@ -19,8 +19,8 @@
 > the full std namespace tail is enabled on top.
 
 **Symptom.** When two modules in one program declare a same-named module
-const with DIFFERENT shapes (scalar `K : s64 : 4` vs array
-`K : [4]s64 : .[...]`), resolution conflates them instead of selecting
+const with DIFFERENT shapes (scalar `K : i64 : 4` vs array
+`K : [4]i64 : .[...]`), resolution conflates them instead of selecting
 per-author:
 
 - **Observed (minimal repro below)**: compiler PANIC — `unresolved type
@@ -30,7 +30,7 @@ per-author:
   own scalar `K` reads as the other module's array global (prints the
   array's address or the whole array). Seen corpus-wide when
   `hash :: #import "modules/std/hash.sx"` (hash.sx declares the SHA-256
-  `K : [64]s64` table) is added to the std.sx namespace tail: examples
+  `K : [64]i64` table) is added to the std.sx namespace tail: examples
   0786/0787/0788/0789/0791/0793/0794 (same-name-const family), 0162, 0168
   all read hash's `K` instead of their own.
 - **Expected**: own-wins / per-author const selection (the documented F2
@@ -45,15 +45,15 @@ consts are robust across every module pulled into every program.
 
 ```sx
 // h.sx
-K : [4]s64 : .[11, 22, 33, 44];
-use_k :: () -> s64 { K[2] }
+K : [4]i64 : .[11, 22, 33, 44];
+use_k :: () -> i64 { K[2] }
 ```
 
 ```sx
 // main.sx
 #import "modules/std.sx";
 h :: #import "h.sx";
-K : s64 : 4;
+K : i64 : 4;
 main :: () { print("K={} h.use_k={}\n", K, h.use_k()); }
 ```
 
@@ -100,7 +100,7 @@ flat-imports std.sx. The panic variant above is the minimal entry point.)
 
 Same-name module consts are selected own-wins via `selectModuleConst`
 (F2, src/ir/lower/expr.zig ~1641) over `module_const_map` — but ARRAY
-consts lower as GLOBALS (`@K = internal global [4 x s64]`), registered in
+consts lower as GLOBALS (`@K = internal global [4 x i64]`), registered in
 a different, still last-wins registry (find it: grep the lowering for
 where a top-level array const becomes a module global — likely
 `lowerGlobalDecl` / the global-var map in src/ir/lower/decl.zig). The