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.

examples/expected/1202-ffi-cc-c-large-aggregate.ir

@@ -96,6 +96,9 @@ declare ptr @cstring(ptr, i64) #0
 ; Function Attrs: nounwind
 declare ptr @int_to_string(ptr, i64) #0
 
+; Function Attrs: nounwind
+declare ptr @uint_to_string(ptr, i64) #0
+
 ; Function Attrs: nounwind
 declare ptr @bool_to_string(ptr, i1) #0
 
@@ -105,6 +108,9 @@ declare ptr @float_to_string(ptr, double) #0
 ; Function Attrs: nounwind
 declare void @hex_group(ptr, ptr, i64, i64) #0
 
+; Function Attrs: nounwind
+declare [4 x i64] @decompose_u16x4(ptr, i64) #0
+
 ; Function Attrs: nounwind
 declare ptr @int_to_hex_string(ptr, i64) #0