feat(bam): add raw byte access for bam::Record

[nh13]

Summary

Add AsRef<[u8]>, TryFrom<Vec<u8>>, and into_inner() to bam::Record, enabling direct
access to the underlying BAM record buffer (without the leading 4-byte block size).

• AsRef<[u8]> — borrow the raw BAM bytes
• TryFrom<Vec<u8>> — construct a Record from raw bytes (validates and indexes the buffer)
• into_inner(self) -> Vec<u8> — consume the record and extract the byte buffer

Motivation

Tools like fgumi need to work with raw BAM record
bytes for performance-critical pipelines (e.g. parallel BGZF block processing, custom
serialization) while still being able to use the alignment::Record trait for field access.
Currently there is no public way to get bytes out of or put bytes into a bam::Record.

This pairs well with the codec encode/decode functions (#364) to give a complete raw-bytes
workflow:

// Encode any Record to raw BAM bytes
let mut buf = Vec::new();
bam::record::codec::encode(&mut buf, &header, &record)?;

// Wrap in a bam::Record — get alignment::Record trait for free
let bam_record = bam::Record::try_from(buf)?;

// Borrow or extract the bytes
let bytes: &[u8] = bam_record.as_ref();
let owned: Vec<u8> = bam_record.into_inner();

Why raw byte access rather than upstreaming all operations?

Tools like fgumi perform three categories of operations on BAM record bytes
that go beyond noodles' current (and appropriate) scope:

1. In-place mutation (tag append/update/remove, base/quality modification,
   flag updates) — bam::Record is intentionally read-only, and a full
   mutation API would be a significant design change
2. Application-specific algorithms (overlap detection, template-coordinate
   sorting, virtual hard clipping) — domain logic that doesn't belong in a
   format library
3. Batched/hot-path accessors (multi-tag extraction in a single pass,
   zero-allocation comparators) — performance-tuned for specific workflows

Raw byte access via AsRef/TryFrom/into_inner is the minimal API that
lets tools like fgumi leverage noodles for I/O and the alignment::Record
trait while performing these operations directly on the buffer.

Test plan

• All existing noodles-bam tests pass (128 unit + 67 doc-tests)
• Doc-tests cover into_inner() and TryFrom<Vec<u8>> round-trip