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.
