Convert FatFS date/time to Unix timestamp in SFTP

[ejohnstown]

• Add TimeTo32_FatFS() to decode FatFS fdate/ftime fields via mktime()
• Use converted timestamp for atime/mtime instead of raw fdate

[ejohnstown]

This should hopefully fix issue #975.

[Copilot]

Pull request overview

This PR updates the SFTP FatFS backend to report file timestamps as Unix epoch seconds instead of returning raw FatFS fdate values, aligning the returned attributes with SFTP’s expected time representation.

Changes:

• Add a FatFS fdate/ftime decoder (TimeTo32_FatFS()) that builds a struct tm and converts it via mktime().
• Populate atr->atime / atr->mtime using the converted Unix timestamp (rather than raw info.fdate).

Comments suppressed due to low confidence (2)

src/wolfsftp.c:4893

• struct tm.tm_year is defined as “years since 1900”, but WS_GETYEAR(d) returns a FatFS year offset from 1980 (0..127). As written this will convert 1980->1900, 2024->1944, etc. Adjust the year by +80 (same as the existing Nucleus TimeTo32() implementation above) before calling mktime().

    tmp.tm_mday = WS_GETDAY(d);
    tmp.tm_mon  = WS_GETMON(d);
    tmp.tm_year = WS_GETYEAR(d);
    tmp.tm_hour = WS_GETHOUR(t);

src/wolfsftp.c:4897

• This adds a new platform-specific date/time conversion that’s easy to get subtly wrong (year base, month base, seconds/2). There is an internal regression test for the analogous Nucleus month conversion (TestNucleusMonthConversion); consider adding a small WOLFSSH_TEST_INTERNAL helper + regress test for FatFS conversion (at least year base + month conversion) so future refactors don’t break timestamps again.

static word32 TimeTo32_FatFS(word16 d, word16 t)
{
    struct tm tmp = {0};

    tmp.tm_mday = WS_GETDAY(d);
    tmp.tm_mon  = WS_GETMON(d);
    tmp.tm_year = WS_GETYEAR(d);
    tmp.tm_hour = WS_GETHOUR(t);
    tmp.tm_min  = WS_GETMIN(t);
    tmp.tm_sec  = WS_GETSEC(t);

    return mktime(&tmp);

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