mem: thread val_ty through inst.Store; per-width comptime regression test

The interp's `storeAtRawPtr` used to write 8 bytes from a `.int` /
`.float` Value regardless of the destination's declared width. The
Value tag flattens s8..s64/u*/pointer all to `.int`, so it can't
disambiguate widths on its own — every store risked clobbering up to
7 neighbor bytes if the actual IR type was sub-8.

Fix:

- `inst.Store` gains `val_ty: TypeId` (defaults to `.void` for
  backward compat with the LLVM emitter, which doesn't read it).
- `builder.store` captures `getRefType(val)` at emit time.
- `storeAtRawPtr` now takes `val_ty`, looks up
  `types.typeSizeBytes(val_ty)`, and writes exactly that many bytes:
  `.int` → width bytes of the i64 representation (1..8),
  `.float` → 4 (f32 round-trip via @floatCast) or 8,
  `.boolean` → 1 (zeros higher width bytes when destination is wider),
  `.null_val` → width bytes of zero. Width outside the expected band
  bails with a clear diagnostic.

Regression test: `examples/132-comptime-typed-store-widths.sx`. For
every primitive type (u8/u16/u32/u64, s8/s16/s32/s64, bool, f32, f64),
the test:

1. Allocates a 32-byte libc buffer through `context.allocator`.
2. Fills with sentinel byte 0xAA.
3. Writes ONE typed value at offset 8.
4. Sums every byte back.
5. Compares the runtime checksum (LLVM-emitted store, already
   correct) against a comptime checksum baked via `#run`.

Mismatch = neighbor clobber. The test exits non-zero with a per-width
"FAIL u8: comptime=X runtime=Y" line so future regressions surface
the offending width.

Also wired:

- Interp's `index_get` gains `.int` / `.byte_ptr` base arms — `buf[i]`
  through a raw libc-malloc'd pointer reads one byte at offset i.
  Used by the new test's `sum_bytes` loop; previously bailed at
  `op=index_get`.
- `emit_llvm`'s comptime-init catch block prints a real diagnostic
  instead of swallowing the error and filling the const with zero.
  Stale bail state from a previous init is cleared before each call.

154/154 example tests pass (the new test + the existing 153). Chess
still green on macOS / iOS sim / Android.

src/ir/module.zig

@@ -329,7 +329,8 @@ pub const Builder = struct {
     }
 
     pub fn store(self: *Builder, ptr: Ref, val: Ref) void {
-        self.emitVoid(.{ .store = .{ .ptr = ptr, .val = val } }, .void);
+        const val_ty = self.getRefType(val);
+        self.emitVoid(.{ .store = .{ .ptr = ptr, .val = val, .val_ty = val_ty } }, .void);
     }
 
     // ── Struct ops ──────────────────────────────────────────────────