Function decode_header 

pub fn decode_header(page: &Page) -> Result<FileHeader>

Decode the page-0 header from the given page buffer. Validates magic, page-size, major version, the per-major format_minor constraint, and the header_crc32c field.

Phase 8 (issue #17): readers accept any SUPPORTED_FORMAT_MAJORS value (0 for pre-1.0 databases, 1 for v1.0+). The per-major format_minor constraint is:

• format_major = 0 → format_minor ∈ {0, 1, 2} (the pre-1.0 incremental rollout: baseline, compression-capable, encryption-capable).
• format_major = 1 → format_minor = 2 (the v1.0 frozen feature-complete value; the only valid minor inside the v1.x series).

Errors

• Error::InvalidFormat if the magic bytes do not match, if format_major is unsupported by this build, if format_minor is not valid for the file’s format_major, or if page_size disagrees with PAGE_SIZE.
• Error::Corruption with page_id = 0 if the stored header_crc32c does not match the CRC32C of the rest of the page.