fix(std): render integer formatter extremes — i64::MIN and unsigned all-ones [F0.8]

Resolves issue 0090. The `{}` integer formatter mis-rendered both ends of
the 64-bit range:

- `int_to_string` computed the magnitude as `0 - n`, which overflows for
  `s64::MIN` (its magnitude is unrepresentable as a positive s64) — the
  value stayed negative, the digit loop ran zero times, so only `-`
  printed. It now extracts digits straight from `n` (per-digit
  `|n % 10|`, `n` truncating toward zero), never negating MIN.

- `any_to_string`'s `case int:` formatted every integer as s64, so a u64
  all-ones value printed as `-1`. There was no `uint` type-category to
  distinguish signedness. Added an additive `type_is_unsigned(T)`
  reflection builtin (static fold + dynamic interp/LLVM paths, mirroring
  `type_name`), backed by the new `TypeTable.isUnsignedInt` predicate, and
  a `uint_to_string` formatter (unsigned decimal via long-division over
  four 16-bit limbs). `case int:` routes through `type_is_unsigned(type)`.

The 16-bit-limb split is factored into a shared `decompose_u16x4`, now
reused by `int_to_hex_string` (no second unsigned-math routine).

Regression: examples/0046-basic-int-formatter-extremes pins both extremes
plus a width spread; unit tests cover `isUnsignedInt`. Docs (specs.md
representation note, readme std API) updated for unsigned/extreme `{}`
behavior. IR snapshots refreshed for the two new std functions.

src/ir/calls.zig

@@ -146,6 +146,7 @@ pub const CallResolver = struct {
             if (std.mem.eql(u8, bare_name, "__trace_resolve_frame"))
                 return refl(bare_name, self.l.module.types.findByName(self.l.module.types.internString("Frame")) orelse .unresolved);
             if (std.mem.eql(u8, bare_name, "is_flags")) return refl(bare_name, .bool);
+            if (std.mem.eql(u8, bare_name, "type_is_unsigned")) return refl(bare_name, .bool);
             if (std.mem.eql(u8, bare_name, "type_of")) return refl(bare_name, .any);
             if (std.mem.eql(u8, bare_name, "field_value")) return refl(bare_name, .any);
             // Generic function — infer return type via type bindings.