stillwater-sc / universal / 26320484701

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feat(elreal): Phase 1 -- block<FpType> representation (#934)

* feat(elreal): Phase 1 -- block<FpType> representation

Phase 1 of the McCleeary LFPERA reimplementation (epic #923). Lands the
foundational `block<FpType>` type with the int32_t `exp_offset` escape
multiplier, the 0-overlap predicate, pretty-printers, and a sweep test
suite parameterised over float, double, half, bfloat16, cfloat<24,5>,
and cfloat<32,8>.

What `block<FpType>` holds
- `FpType v`: sign + biased exponent + k-bit significand packed into the
  host FP type.
- `int32_t exp_offset`: multiplier in units of `2^E_FpType` so a single
  block can carry an exponent far outside the host type's hardware range,
  which Phase 7 transcendentals will need.

Effective value `v * 2^(exp_offset * 2^E_FpType)`.
Combined exponent `E(b) = scale_of(v) + exp_offset * 2^E_FpType`.
McCleeary's k = numeric_limits<FpType>::digits.

Files
- `include/sw/universal/number/elreal/exp_field_width.hpp`: trait that
  yields E for float (8), double (11), bfloat16 (8), cfloat<N,E> (E).
- `include/sw/universal/number/elreal/block.hpp`: block<FpType> struct
  plus `zero_overlap(b1, b2)` free function. Trivial layout (no in-class
  initialisers), branches on `has_universal_fp_api_v<FpType>` to pick
  between Universal's .sign()/.scale() and native std::signbit/std::ilogb.
- `include/sw/universal/number/elreal/block_manipulators.hpp`: to_binary,
  to_hex, color_print. to_hex memcpys the bit pattern for native float
  and double; for Universal wrappers it falls back to the human-readable
  form (to avoid cross-header churn -- richer rendering can come later).
- `include/sw/universal/number/elreal/elreal.hpp`: umbrella header.
- `include/sw/universal/number/elreal/elreal_fwd.hpp`: forward decl.

Tests under `elastic/elreal/block/`
- construction.cpp: trivial-copyable + trivial-destructible assertions;
  value and value+offset construction; copy semantics; compile-time
  constants (k, E, exp_step).
- accessors.cpp: sig... (continued)

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