Diagnostics: native crashes, early crash log, report keeps the file

[Qwinty]

Follow-up to #1, closing the gaps found when reviewing it.

What changes

• Native crashes leave a trace. The panic hook only saw Rust panics, while the likely crash in this app is an access violation in Media Foundation, D3D11 or Win32. An unhandled-exception filter now writes last-crash.log with the exception code, address and module (native exception 0xC0000005 at 0x… in mfplat.dll), then the in-memory log. It allocates nothing (the heap may be broken), then calls the previous filter and lets Windows Error Reporting run as before.
• Crashes before setup are kept. diag::init works out the log folder itself (logs\ next to portable data, or %LOCALAPPDATA%\io.github.plugcam\logs like Tauri's app_log_dir) and creates it. A test checks the identifier against tauri.conf.json.
• The crash handlers wait for the logger lock (up to 100 ms) instead of dropping the crash when another thread is mid-line.
• The report keeps the detailed log after it is turned off. Before, turn on → reproduce → turn off → Save report lost the file. Now the report has plugcam.log whenever it exists (titled with when it was last written), plus this session's lines from memory.
• detailed log off goes into the file before it closes, so the file shows where it ends.
• The report lists each GPU once (DXGI listed the Radeon 780M twice here).

Checks

• cargo test --lib: 57 passed (4 new); cargo clippy --all-targets: no new warnings.
• Crash probe (a throwaway example, not committed) with LOCALAPPDATA pointed at a temp folder: a null write produced native exception 0xC0000005 at … in zz_crash_probe.exe plus the buffer; a thread panic produced the panic and the buffer.
• Real app (portable debug build, driven through WebView2 DevTools): Settings → Diagnostics renders; turning the detailed log on creates plugcam.log with the session's earlier lines; off writes the last line; Save report writes the file to Downloads with both log sections, one GPU, the phone serial masked (***e2d7, including the Wi-Fi ADB mDNS names) and no profile path.

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