SRombauts / SQLiteCpp / 28478122417

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Fix Database::getHeaderInfo() signed-shift UB and use fixed-width Header types (#558)

## Summary

Fixes two findings from the deep code review (DB-02, DB-03) in
`Database::getHeaderInfo()` and the public `Header` struct.

### DB-02 — signed left-shift UB in the big-endian assembly
The multi-byte header fields were assembled from `unsigned char`
operands, which promote to `int` before the `<< 24` shift. For any byte
with the high bit set this is signed-left-shift undefined behaviour, and
the `int` result was then sign-extended when widened to the (LP64)
`unsigned long` fields.

Each byte is now cast to `std::uint32_t` before shifting, through a
small local `readBE32` helper that also collapses the 14 repeated 4-line
blocks.

### DB-03 — platform-variable field widths
`Header` used `unsigned long`, which is 32-bit on LLP64 (Windows) but
64-bit on LP64 (Linux/macOS), so the struct layout and field widths
differed across platforms. The struct now uses fixed-width `<cstdint>`
types (`std::uint8_t` / `std::uint32_t`).

## Test plan
- Built `SQLiteCpp_tests` (MSVC, Debug) — clean.
- `ctest -C Debug` — 100% passed, including `Database.getHeaderInfo`.

No public API behaviour changes; the parsed values are identical, only
the field types and the (previously UB) assembly path change.

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639 of 641 relevant lines covered (99.69%)

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