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feat(internal): expansion limb trait and expansion_ops templated on FpType (#1569)

* feat(internal): expansion limb trait and expansion_ops templated on FpType

Part 1 of #1355: the expansion arithmetic under ereal works on any limb
type whose arithmetic is a p-bit IEEE-754 binary format -- float, double,
x87 extended, binary128 -- not just double.

- is_expansion_limb_v<T> decides what may be a limb. is_iec559 alone does
  not: libstdc++ reports it true for IBM double-double, the default
  ppc64le long double (measured under QEMU), on which two_sum is not
  error-free. The added test is IEEE's emin = 1 - emax, i.e.
  min_exponent == 3 - max_exponent, which double-double fails (-968 vs
  1024). two_sum, fast_two_sum and two_prod static_assert it, with the
  reason and the -mabi=ieeelongdouble way out in the message.
- Every function in expansion_ops is a template on the limb type,
  defaulted to double so braced-list calls keep resolving as before.
- What was double's is derived from the limb type: the #1553 range
  thresholds (1000, 1020, and -1100 for zero) from max_exponent and
  min_exponent, the 53-bit overlap checks from digits.

The double path is bit-identical: a differential harness over every
operation (4000 seeded cases, including the top of the range) produces
byte-identical output from the old and new header, on gcc and clang.

A side effect: expansion_ops' EFTs no longer collide with floatcascade's,
which declares plain-double ones in the same namespace, so a program can
now use ereal together with dd_cascade, td_cascade or qd_cascade -- that
was a redefinition error before.

Relates to #1355
Resolves #1563

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* fix(internal): keep the limbs range scaling cannot carry

The range management of #1553 scales both operands down before a sum or
product near the top of the range. It scaled every component, and a
component whose low bits fall below the smallest subnormal once sh... (continued)

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