add atomic saving via .tmp files and ReplaceFileW for Windows

[nschimme]

Summary by Sourcery

Add atomic file saving support using temporary files and platform-specific rename semantics, including proper fsync handling on Windows.

Bug Fixes:

• Ensure file data is reliably flushed to disk on Windows by implementing fsync via FlushFileBuffers.
• Perform atomic renames on Windows using ReplaceFileW with a MoveFileExW fallback when the target file does not exist.

Enhancements:

• Unify temporary file save behavior across platforms by always writing to .tmp files and renaming them via a shared io::rename helper.

[sourcery-ai]

Reviewer's Guide

Adds atomic file saving support via temporary .tmp files, implementing a cross‑platform io::rename (using ReplaceFileW/MoveFileExW on Windows and ::rename elsewhere) and wiring FileSaver to always write to a .tmp and then atomically rename, while also making fsync() functional on Windows via FlushFileBuffers and exception‑based error reporting.

Sequence diagram for atomic file saving with tmp files and io_rename

sequenceDiagram
    actor User
    participant FileSaver
    participant QFile
    participant io
    participant OS

    User->>FileSaver: save(targetFilename)
    FileSaver->>FileSaver: tmpName = targetFilename + .tmp
    FileSaver->>QFile: open(tmpName, WriteOnly)
    FileSaver->>QFile: write(data)
    FileSaver->>QFile: flush()
    FileSaver->>io: fsync(file)
    io->>OS: FlushFileBuffers or fsync
    OS-->>io: success or error
    io-->>FileSaver: return or throw IOException
    FileSaver->>io: rename(tmpName, targetFilename)
    io->>OS: ReplaceFileW or MoveFileExW or rename
    OS-->>io: success or error
    io-->>FileSaver: return or throw IOException
    FileSaver-->>User: save completed or error

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Class diagram for io_namespace helpers and FileSaver tmp_rename integration

classDiagram
    class io {
        +bool fsync(QFile file) CAN_THROW
        +IOResultEnum fsyncNoexcept(QFile file) noexcept
        +void rename(QString from, QString to) CAN_THROW
    }

    class FileSaver {
        -QFile device
        -QString filename
        +~FileSaver()
        -static QString maybe_add_suffix(QString filename)
        -static void remove_tmp_suffix(QString filename) CAN_THROW
    }

    io <.. FileSaver : uses
    FileSaver : maybe_add_suffix returns filename + ".tmp"
    FileSaver : remove_tmp_suffix calls io.rename(from, to)

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File-Level Changes

Change	Details	Files
Implement functional fsync() on Windows using FlushFileBuffers with proper error propagation.	Include <io.h> and <windows.h> when building for Windows so low-level APIs are available. Change Windows fsync(QFile&) implementation from a stub that returns false to a real implementation calling ::_get_osfhandle and ::FlushFileBuffers. Throw io::IOException with the Windows error code from ::GetLastError() if FlushFileBuffers fails, aligning error handling with POSIX branches.	src/global/io.cpp
Introduce io::rename() as a cross-platform atomic rename abstraction with Windows-specific behavior using ReplaceFileW/MoveFileExW.	Add io::rename(const QString&, const QString&) in io.cpp with Windows specialization using std::wstring, ReplaceFileW for atomic replacement, and a MoveFileExW fallback when the target does not exist. Use QFile::encodeName plus ::rename on non-Windows platforms and translate failures into io::IOException::withCurrentErrno(). Declare the new rename function in io.h for shared use across the codebase.	src/global/io.cpp src/global/io.h
Make FileSaver always save via a .tmp file and then atomically rename it into place using io::rename.	Remove the platform-dependent USE_TMP_SUFFIX flag so that all platforms use the .tmp suffix for temporary save files. Have maybe_add_suffix() unconditionally append ".tmp" to the target filename to form the temporary path. Change remove_tmp_suffix() to call io::rename(from, to) instead of directly invoking ::rename, and construct from/to as filename+".tmp" and filename, respectively, centralizing atomic rename behavior.	src/mapstorage/filesaver.cpp

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[sourcery-ai]

Hey - I've left some high level feedback:

• In the Windows-specific fsync implementation, consider explicitly checking for an invalid file handle (e.g., handle == -1) and throwing a clearer IOException before calling FlushFileBuffers, so failures are easier to diagnose and don't rely on the Win32 error alone.
• In io::rename on Windows, you may want to include REPLACEFILE_WRITE_THROUGH in the ReplaceFileW flags to better mirror the durability guarantees expected from an atomic save operation in the presence of power loss.
• The fallback from ReplaceFileW to MoveFileExW with MOVEFILE_COPY_ALLOWED can lose atomicity when the destination does not yet exist, especially across volumes; if atomic replacement semantics are required, it may be safer to detect cross-volume moves and handle them differently or fail explicitly.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- In the Windows-specific `fsync` implementation, consider explicitly checking for an invalid file handle (e.g., `handle == -1`) and throwing a clearer `IOException` before calling `FlushFileBuffers`, so failures are easier to diagnose and don't rely on the Win32 error alone.
- In `io::rename` on Windows, you may want to include `REPLACEFILE_WRITE_THROUGH` in the `ReplaceFileW` flags to better mirror the durability guarantees expected from an atomic save operation in the presence of power loss.
- The fallback from `ReplaceFileW` to `MoveFileExW` with `MOVEFILE_COPY_ALLOWED` can lose atomicity when the destination does not yet exist, especially across volumes; if atomic replacement semantics are required, it may be safer to detect cross-volume moves and handle them differently or fail explicitly.

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 25.40%. Comparing base (dd0c268) to head (2b78e99).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/global/io.cpp	0.00%	5 Missing ⚠️
src/mapstorage/filesaver.cpp	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #527      +/-   ##
==========================================
- Coverage   25.40%   25.40%   -0.01%     
==========================================
  Files         519      519              
  Lines       43109    43113       +4     
  Branches     4699     4699              
==========================================
  Hits        10952    10952              
- Misses      32157    32161       +4     

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