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/0112-int-literal-out-of-range-silent-wrap.md

@@ -18,14 +18,14 @@ Coverage via the shared arm: decls, assignments, call args, struct-literal
 fields, struct constants, globals.
 
 **Behavior change:** `examples/0300-closures-lambda.sx` passed `133` to an
-`s3` param and pinned the wrapped `-3`; updated to a fitting value.
+`i3` param and pinned the wrapped `-3`; updated to a fitting value.
 
 **Regression tests:** `examples/1156-diagnostics-int-literal-out-of-range.sx`
 (both faces diagnosed in one run) and
 `examples/0174-types-int-literal-boundaries.sx` (extreme in-range values,
 width-64 types, `xx`/`cast` escapes, call args).
 
-**Found during the fix:** negated-literal GLOBAL initializers (`g : s64 = -1;`)
+**Found during the fix:** negated-literal GLOBAL initializers (`g : i64 = -1;`)
 are rejected as non-constant — pre-existing gap, filed as issue 0113.
 
 ---
@@ -33,10 +33,10 @@ are rejected as non-constant — pre-existing gap, filed as issue 0113.
 # 0112 — out-of-range int literal silently wraps into a narrower annotated target
 
 **Symptom.** An integer literal that does not fit its explicitly-annotated
-integer target truncates with no diagnostic: `x : s8 = 300;` binds 44,
+integer target truncates with no diagnostic: `x : i8 = 300;` binds 44,
 `y : u8 = 256;` binds 0. Expected: a compile-time error (the value is known
 exactly at compile time; this is the integer analogue of the float→int
-narrowing rule, which errors on non-exact `y : s64 = 1.5`).
+narrowing rule, which errors on non-exact `y : i64 = 1.5`).
 
 Split from issue 0111 (whose fix removed the *implicit* narrowing — an
 unannotated `x := 0` no longer adopts the fn return type — but the explicit
@@ -48,7 +48,7 @@ annotation path keeps wrapping).
 #import "modules/std.sx";
 
 main :: () {
-    x : s8 = 300;
+    x : i8 = 300;
     print("x: {}\n", x);
     y : u8 = 256;
     print("y: {}\n", y);
@@ -58,7 +58,7 @@ main :: () {
 - **Observed** (current master): prints `x: 44` / `y: 0`, exit 0, no
   diagnostic.
 - **Expected**: compile error per literal, e.g.
-  `integer literal 300 does not fit in s8 (range -128..127)`, and the analog
+  `integer literal 300 does not fit in i8 (range -128..127)`, and the analog
   for `256` / `u8 (range 0..255)`.
 
 Repro co-located: `issues/0112-int-literal-out-of-range-silent-wrap.sx`
@@ -72,7 +72,7 @@ value truncates at LLVM emission width. The annotated-decl path
 (`lowerVarDecl` with `type_annotation`, `src/ir/lower/stmt.zig` ~255) sets
 `target_type` to the annotation before lowering the initializer, so every
 annotated narrow decl funnels through this arm. Assignments to narrow
-lvalues (`b = 300` where `b: s8`) reach the same arm via `lowerAssignment`'s
+lvalues (`b = 300` where `b: i8`) reach the same arm via `lowerAssignment`'s
 LHS-derived target and likely need the same check.
 
 ## Investigation prompt (paste into a fresh session)
@@ -84,15 +84,15 @@ LHS-derived target and likely need the same check.
 > table knows both; mirror the bounds logic used by
 > `TypeResolver.integerLimitFor`). On overflow emit a diagnostic via
 > `self.diagnostics.addFmt(.err, node.span, ...)` naming the literal, the
-> type, and its range — do NOT silently fall back to s64 (REJECTED PATTERNS:
+> type, and its range — do NOT silently fall back to i64 (REJECTED PATTERNS:
 > no silent fallback defaults); still return a `constInt` of the target type
 > so lowering continues to surface further errors. Audit sibling literal
 > sinks that bypass this arm (comptime folds, `lowerStructConstant`, global
 > initializers) for the same check.
 >
 > Verify: `issues/0112-int-literal-out-of-range-silent-wrap.sx` errors with
-> two diagnostics (s8/300, u8/256); boundary values still compile
-> (`x : s8 = -128` / `127`, `y : u8 = 0` / `255`, `m : u64` large literals).
+> two diagnostics (i8/300, u8/256); boundary values still compile
+> (`x : i8 = -128` / `127`, `y : u8 = 0` / `255`, `m : u64` large literals).
 > `zig build && zig build test && bash tests/run_examples.sh` — any example
 > that relied on silent wrapping must be reviewed individually. Promote the
 > repro per the resolution flow (likely `examples/11xx-diagnostics-...`).