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.

library/modules/std/process.sx

@@ -21,12 +21,12 @@ libc :: #library "c";
 // ── Low-level libc bindings ─────────────────────────────────────────
 
 popen  :: (cmd: [:0]u8, mode: [:0]u8) -> *void #foreign libc;
-pclose :: (stream: *void) -> s32 #foreign libc;
+pclose :: (stream: *void) -> i32 #foreign libc;
 fread  :: (ptr: [*]u8, size: usize, nmemb: usize, stream: *void) -> usize #foreign libc;
-feof   :: (stream: *void) -> s32 #foreign libc;
+feof   :: (stream: *void) -> i32 #foreign libc;
 getenv :: (name: [:0]u8) -> *u8 #foreign libc;
 strlen :: (s: *u8) -> usize #foreign libc;
-system :: (cmd: [:0]u8) -> s32 #foreign libc;
+system :: (cmd: [:0]u8) -> i32 #foreign libc;
 
 // ── Public types ─────────────────────────────────────────────────────
 
@@ -34,7 +34,7 @@ ProcessResult :: struct {
     /// Exit code as reported by `WEXITSTATUS(status)`. 0 = success.
     /// Note: doesn't distinguish "killed by signal" from "exited
     /// non-zero"; phase 1B will return a tagged union.
-    exit_code: s32;
+    exit_code: i32;
     stdout:    string;
 }
 
@@ -50,7 +50,7 @@ ProcessResult :: struct {
 // with stdout, append " 2>&1" to the command.
 run :: (cmd: [:0]u8) -> ?ProcessResult {
     f := popen(cmd, "r");
-    if cast(s64) f == 0 { return null; }
+    if cast(i64) f == 0 { return null; }
 
     out := "";
     buf := cstring(4096);
@@ -61,7 +61,7 @@ run :: (cmd: [:0]u8) -> ?ProcessResult {
         if n > 0 {
             chunk : string = ---;
             chunk.ptr = buf.ptr;
-            chunk.len = cast(s64) n;
+            chunk.len = cast(i64) n;
             out = concat(out, chunk);
         }
     }
@@ -85,12 +85,12 @@ run :: (cmd: [:0]u8) -> ?ProcessResult {
 // string if set to "".
 env :: (name: [:0]u8) -> ?string {
     p := getenv(name);
-    addr : s64 = xx p;
+    addr : i64 = xx p;
     if addr == 0 { return null; }
     n := strlen(p);
     if n == 0 { return ""; }
-    buf := cstring(cast(s64) n);
-    memcpy(buf.ptr, xx p, cast(s64) n);
+    buf := cstring(cast(i64) n);
+    memcpy(buf.ptr, xx p, cast(i64) n);
     buf
 }
 
@@ -126,7 +126,7 @@ find_executable :: (name: [:0]u8) -> ?string {
 // via `write(2)`, so skipping the stdio flush loses nothing. Binding the
 // symbol `"exit"` would also collide with this module's own `exit` function
 // at the link level.
-clib_exit :: (code: s32) -> noreturn #foreign libc "_exit";
+clib_exit :: (code: i32) -> noreturn #foreign libc "_exit";
 
 // Stop the process immediately with exit code `code`. Does NOT unwind:
 // no `defer` / `onfail` cleanup runs, no error-trace frames are pushed —