[Session] Truncate session file after successful write - #278
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The files save handler ftruncated the session file to 0 before writing, so a failed or short write returned FAILURE with the previous session data already destroyed. Write first and truncate the old tail to the new length only after the full buffer was written successfully. Sibling audit: PS_WRITE_FUNC/PS_UPDATE_FUNC callers and the read path are unaffected; the empty-write destroy path truncates identically.
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The files session save handler truncated the session file to 0 before writing new data, so a failed or short write (RLIMIT_FSIZE with SIGXFSZ ignored, for example) made ps_files_write() return FAILURE while the previous session data was already gone. ps_files_write() now writes first and truncates to the new length only after the full buffer lands, so failed writes leave prior data intact. A regression test reproduces the loss via posix_setrlimit(POSIX_RLIMIT_FSIZE); PS_WRITE_FUNC, PS_UPDATE_TIMESTAMP_FUNC and PS_DESTROY_FUNC were audited and behave identically. Closes aph-er5.