fix: correct length field size, reserved tag range, and flaky test assertion

[immanuwell]

Description

Few small inaccuracies found while reading the code:

• Vortex doc comment says Length field is 8 bytes, but it's put_u32 (4 bytes). Same comment says max value is 1 GiB but MAX_VALUE_SIZE is 4 GiB.
• From<Vortex> for Bytes impl has a copy-paste comment saying From<PieceContent>.
• test_header_new asserts header.id <= 254, but rng.gen::<u8>() produces 0-255, so it fails ~1/256 runs when id == 255.
• Reserved tag comment says "for tags 6-254", correct range is 8-253 (6 and 7 are defined, 254 is Close). Same issue in docs/README.md.

Related Issue

No open issue - doc/comment/test fixes only.

Motivation and Context

Flaky test repro: run cargo test tests::test_header_new repeatedly, panics when id == 255.