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fix(native): setFields(long double) reconstructs the explicit integer bit (#1262) (#1268)

* fix(native): setFields(long double) reconstructs the explicit integer bit (#1262)

On the x86/RISC-V 80-bit extended path, setFields(long double) had two problems:
its fraction mask 0xFFFF'FFFF'FFFF'FFFF was a no-op (the target parts.fraction is
a 63-bit field, so the assignment truncated implicitly -> -Wconversion / MSVC
C4244), and -- the real defect -- it never set the EXPLICIT integer bit
(parts.bit63), which must be 1 for every value with a nonzero exponent. As a
result the extractFields->setFields round-trip produced NaN for EVERY normal
value (verified: 1.0L, 2.0L, pi, ... all decoded to NaN).

Fix (both non-DOWNCAST copies): mask the fraction to the 63-bit field explicitly
and set parts.bit63 = (exponent != 0). Guarded to the layouts that actually have
the 63-bit-fraction + bit63 fields (UNIVERSAL_ARCH_X86_64 / _RISCV); other
long double layouts (binary128 / double-double, no bit63 field) keep the original
assignment unchanged. Confirmed the issue's open question: the field is NOT too
narrow -- the integer bit has its own field -- so the fix belongs in setFields.

Regression: static/native/float/ieee754.cpp already has a generic
VerifyFieldRoundTrip<Real>; instantiate it for long double, scoped to the 80-bit
x86/RISC-V path (the uint64 field interface can't round-trip a 128-bit fraction,
and the DOWNCAST path is lossy for >53-bit values -- both independent of #1262).

gcc + clang (both non-DOWNCAST on x86_64 here): float/double/long double field
round-trip PASS; full native_float_ieee754 PASS.

Resolves #1262

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* ci: allow 'native' scope in conventional-commit PR-title lint

The native/ subsystem (ieee754 helpers: extractFields/setFields/etc.) is a
legitimate recurring scope but was missing from the allow-list, so
fix(native): ... PR titles failed lint-pr-title (#1262 / PR #1268).

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