Update modified time when closing written files

[Old-Ding]

Closes #81

Summary

• Add ffconfigUPDATE_FILE_MODIFIED_TIME_ON_CLOSE, defaulting to 0 to preserve existing close-time behavior.
• Track actual file changes with FF_VALID_FLAG_MODIFIED after successful writes or EOF truncation.
• Reuse the existing FF_Close() directory-entry update to refresh xModifiedTime when the opt-in config is enabled.

Testing

• git diff --check upstream/main...HEAD
• Verified touched files are UTF-8 without BOM and use CRLF line endings.
• Not run: cmake -S test/unit-test -B test/unit-test/build/ because cmake is unavailable on this Windows host; Ruby and a C compiler are also unavailable locally.

[Old-Ding]

Follow-up verification:

• The implementation follows the direction discussed in [BUG] FF_Close() does not update xModifiedTime with current system time when file is modified (ffconfigTIME_SUPPORT=1) #81: the new behavior is opt-in through ffconfigUPDATE_FILE_MODIFIED_TIME_ON_CLOSE, defaulting to 0 to preserve existing close-time behavior.
• The timestamp update reuses the existing directory-entry write in FF_Close(), so it does not add a second FF_GetEntry() / FF_PutEntry() cycle for modified files.
• Unit tests were added for the unchanged-size modified-file case and the unmodified-file case, and git diff --check upstream/main...HEAD passes.

I still do not have CMake/Ruby/a C toolchain available on this Windows host to run the new unit test locally.