Use fixed-width types in SQLite::Header

[SRombauts]

Summary

• use std::uint8_t and std::uint32_t for SQLite::Header fields
• replace the 3.x ABI guard with compile-time checks for all fixed-width public field types
• document the intentional 4.0 ABI change

Rationale

This reintroduces the public type change from #558 that #575 intentionally deferred to preserve the 3.4.0 ABI.

SQLite database-header fields have defined byte widths. Using fixed-width integer types makes those widths independent of the platform data model, notably avoiding 64-bit unsigned long fields on LP64 systems. The existing UB-free big-endian parsing remains unchanged.

Testing

The existing Database.getHeaderInfo test covers the parsed values. Compile-time assertions cover every public Header field type.

The same code change previously passed the full CI matrix. The branch has now been rebased onto master after the shared-library ABI generation was bumped from 0 to 1 in both CMake and Meson, and CI is revalidating that final state.

[coveralls]

coverage: 99.715%. remained the same — abi-header-fixed-width-types into master