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/0054-generic-struct-to-param-protocol-erasure.md

@@ -5,7 +5,7 @@
    was a general pre-existing bug — `self.x` failed on *any* generic-struct
    impl method.)
 2. `createProtocolThunk` monomorphizes the template method for a generic-struct
-   instance (`Combined.get` → `Combined__s64_s64.get` with the instance
+   instance (`Combined.get` → `Combined__i64_i64.get` with the instance
    bindings), so the erasure vtable dispatches instead of an `unreachable` thunk.
 
 `xx c` (Combined → VL($R)) now dispatches correctly. The *full* canonical `map`
@@ -27,18 +27,18 @@ This is the last piece of the canonical `map` (`return xx c;`).
 ```sx
 #import "modules/std.sx";
 VL :: protocol(T: Type) { get :: () -> T; }
-IntCell :: struct { v: s64; }
-impl VL(s64) for IntCell { get :: (self: *IntCell) -> s64 => self.v; }
+IntCell :: struct { v: i64; }
+impl VL(i64) for IntCell { get :: (self: *IntCell) -> i64 => self.v; }
 Combined :: struct($R: Type, ..$Ts: []Type) { sources: (..VL(Ts)); value: $R; }
 impl VL($R) for Combined($R, ..$Ts) { get :: (self: *Combined) -> $R => self.value; }
 
-make :: (..sources: VL) -> VL(s64) {
-    c : Combined(s64, ..sources.T) = ---;
+make :: (..sources: VL) -> VL(i64) {
+    c : Combined(i64, ..sources.T) = ---;
     c.value = 99;
     c.sources = (..sources);
-    return xx c;                 // Combined__s64_s64 -> VL(s64)
+    return xx c;                 // Combined__i64_i64 -> VL(i64)
 }
-main :: () -> s32 {
+main :: () -> i32 {
     r := make(IntCell.{ v = 1 });
     print("{}\n", r.get());      // expect 99; instead traps
     0;
@@ -53,11 +53,11 @@ diagnostic — so an impl *was* matched), but the JIT traps on `r.get()`.
 `param_impl_map` is keyed by **concrete** `(protocol, target_args_mangled,
 source_mangled)`. The impl `impl VL($R) for Combined($R, ..$Ts)` is generic on
 both sides — its source mangles to a generic `Combined` (with `$R`/`$Ts`), not
-the concrete `Combined__s64_s64`. Erasing `Combined__s64_s64 → VL(s64)` looks up
-`(VL, s64, Combined__s64_s64)`, which doesn't key-match the generic impl; some
+the concrete `Combined__i64_i64`. Erasing `Combined__i64_i64 → VL(i64)` looks up
+`(VL, i64, Combined__i64_i64)`, which doesn't key-match the generic impl; some
 looser path still produces a protocol value, but its vtable slot for `get`
-isn't bound to the monomorphized `Combined__s64_s64.get` (which returns
-`self.value` as `$R`=s64). Calling through it traps.
+isn't bound to the monomorphized `Combined__i64_i64.get` (which returns
+`self.value` as `$R`=i64). Calling through it traps.
 
 The fix needs generic-impl matching + per-instance monomorphization for protocol
 erasure: when erasing a concrete generic-struct instance to a parameterized
@@ -75,7 +75,7 @@ and fill the vtable with the resulting fn-ptrs. Compare:
 # Verification
 
 The reproduction should print `99`. Plain (non-generic) struct → parameterized
-protocol erasure already works (`examples/206`: `xx IntCell -> VL(s64)`); the gap
+protocol erasure already works (`examples/206`: `xx IntCell -> VL(i64)`); the gap
 is specifically a *generic-struct* source matched via a *generic* impl.
 
 # Status