skill: the five failure modes this library run actually produced

Every one of these cost real time today and each has a detector now: RaiDrive returning success
on a truncated write (12.5 MB of 403 MB, hash read-back caught it), the share's toasts being an
overload signal rather than a permissions problem (3 concurrent jobs), a picture gate comparing
by frame index after an input seek (false FAIL at min SSIM 0.25 on a good encode; fixed by
trimming on PTS), frame-count differences meaning 'gate not applicable' instead of 'broken file',
a filename with a space before its extension being innocent, and killing encoders by process name
(which took out an unrelated pilot encode).

SKILL.md

@@ -260,6 +260,13 @@ re-encoded to `-b:a 256k`: that added ~128 kbps for no audible gain, ~+10% file
 
 ## Pitfalls
 
+- **A RaiDrive/Dokan write can return with NO error while the file is a fraction of what it should be.** Measured: a 403 MB upload left 12.5 MB on the share, and RaiDrive's own "the operation didn't complete within specific time limit" toast arrived *after* the write had already returned success. Never trust a write to a Dokan-backed share: hash the local file, hash the share copy back, compare size AND SHA-256, and only then rename. On a mismatch, retry the upload (it usually works on the second attempt) rather than failing the item. Uploads should use small chunks (4 MB) with a periodic `flush`+`fsync`; a partial `*.new.mp4`/`*.part.mp4` must be deleted, never resumed from `os.path.getsize()` on the share (RaiDrive reports stale sizes, so a resume offset taken from it corrupts the file).
+- **Those same toasts are an overload signal, not a permission problem.** Three jobs reading/writing one share provoked both the truncated write above and `[Errno 9] Bad file descriptor` on a 1 GB staged read; writing into the library folder itself was perfectly fine (verified by creating and deleting a file there). Cap the concurrency at two jobs (one CPU tier + one GPU tier), give each tier its own lock file, and make the lock take over a stale one (pid gone).
+- **Compare pictures by TIMESTAMP, never by frame index after an input seek.** An input `-ss` snaps to a keyframe, and it snaps to a *different* keyframe for the source and for the encode - measured: two 1500-frame decodes whose start times differed by ~10 s, so every frame met the wrong one and a perfectly good encode scored min SSIM 0.25. Cut the window with the `trim` filter on absolute PTS (`-copyts -ss <before window> -i in -vf "trim=start=X:end=Y,setpts=PTS-STARTPTS,..."`). Validate the method by comparing a file against ITSELF: it must read lo=1.0/mean=1.0, otherwise the harness is broken and every verdict it produces is noise.
+- **If the frame counts in the sampled window differ by more than ~1%, the picture gate is NOT APPLICABLE - do not fail the file.** A source with holes in its timeline (measured: 1459 frames where 1500 belong, a 1.7 s hole) is *supposed* to gain duplicated frames from `-fps_mode cfr`; after the hole every frame pairs with the wrong one and SSIM collapses. Report the condition and let the structural/decode/size gates carry the verdict.
+- **File paths with a space before the extension are NOT the problem they look like** - `1130 维特根斯坦十讲 .mp4` opened and streamed fine. Suspect the share, not the name.
+- **Never kill encoders by process name.** `Get-Process ffmpeg | Stop-Process -Force` also killed an unrelated pilot encode that was running legitimately (rc=-1 mid-encode). Kill the specific background PID, or the specific `ffmpeg` PID you started.
+
 - **Never write the encode output directly to a network share.** A streaming `.part` write over
   SMB/SFTP/Dokan leaves a corrupt stub when the connection blips. Encode to local disk, verify,
   then copy the complete file across. The tool writes `dst + ".part.mp4"` and renames only after