replace lossy 2-bit truncation with rotation-based encoding for
32-bit + MRB_USE_FLOAT32, matching the technique used for 64-bit
float64. rotl32(bits - ADDEND, 3) maps biased exponents [95, 158]
(actual [-32, +31]) to properly tagged inline values with zero
precision loss. special values (0, Inf, NaN) use sentinel constants;
out-of-range floats fall back to heap-allocated RFloat.
also fix a pre-existing alignment issue: RVALUE was 20 bytes on
32-bit, causing 4-byte-aligned objects to be misidentified as
immediates by word boxing (WORDBOX_IMMEDIATE_MASK=0x07 requires
8-byte alignment). pad RVALUE to 24 bytes on 32-bit + float32.
Co-authored-by: Claude <noreply@anthropic.com>
Yukihiro "Matz" Matsumoto
14a5cfb87f