Fix bitfields spanning nine storage bytes

[tamird]

496817f replaced bit-at-a-time access with byte loops that assemble
the field in a u64. A valid 64-bit field starting one bit into storage
spans nine bytes, so get(1, 64) and set(1, 64, value) shift by 64 on the
last iteration. The setter also discards the high bit when aligning the
value. The existing full-word regression test covered aligned fields and
fields ending at bit 64, leaving this crossing case untested.

Handle the ninth byte separately, after shifting the first word on reads
and before shifting away the high bits on writes. Share the read and
write implementations across runtime, raw-pointer, and const-generic
accessors instead of maintaining separate usize and u64 algorithms. The
const-generic wrappers pass their constants into the inlineable helpers,
and the getters remain usable in constants.

Store byte-aligned 64-bit fields directly with write_unaligned. Such
fields use native byte order and need no read-modify-write. This keeps
AArch64 code generation to a single store instead of vectorizing eight
byte extractions into shifts, shuffles, and constant-pool loads.

Use byte pointers without creating references to the whole storage, so
raw access can leave neighboring bytes uninitialized. Check the field's
bounds before accessing storage, including in release builds, to
preserve the safe accessors' bounds checking and reject invalid writes
before modifying any bytes.

Replace comparisons between accessors sharing the same algorithm with
bit-at-a-time expected values. Cover narrow and full-width fields at
every intra-byte offset, preservation of neighboring bits, constant
evaluation, indirect storage, and bounds failures. Enable this coverage
on both endiannesses while retaining the older little-endian-specific
cases behind their existing condition.

[tamird]

r? @jsgf @emilio

This was written by LLM and reviewed by me. The bug was also spotted by multiple LLMs in aya-rs/aya#1730.

[rustbot]

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[tamird]

r? @emilio

[tamird]

Another note: this diff looks much cleaner with --diff-algorithm=histogram.