fix(dav): upsert upload matches server-converted ext (xls→xlsx) (#82)

OnlyOffice converts legacy binary Office uploads (.xls/.doc/.ppt) into
OOXML (.xlsx/.docx/.pptx) on the server. UploadToFolderReplacing matched
by the exact stem|ext via FindFilesByDedupKey, so a repeated
`oo dav upload FOLDER f.xls --replace` never found the stored f.xlsx and
appended a second file (live: ids 3799+3887, 3800+3888).

- EquivalentUploadExt / FindFilesByStemExt: match by stem with a
  legacy↔OOXML extension equivalence, so .xls finds the saved .xlsx.
- planUploadReplacement: pick the surviving file and the redundant
  duplicate ids for a replacing upload.
- UploadToFolderReplacing updates the existing file in place (UpdateFile,
  stable id, no delete window), removes extra duplicates, and falls back
  to conversion-aware delete + upload when the portal rejects the update.
- AssertNoFileConflict (--no-replace) uses the same conversion-aware
  matching so a .xls upload conflicts with an existing .xlsx.
- Offline tests cover the matcher, the plan and the repeated-.xls
  regression; pdf/xlsx behaviour unchanged.

README.md

@@ -743,7 +743,7 @@ oo dav ls @root                       # virtual sections (Documents, Projects,
 oo dav mkdir 659 "2026 inbox"
 oo dav ensure-path "Banks/Caixa"      # resolve-or-create; idempotent, prints the folder id
 oo dav ensure-path "Banks/Caixa" --under 659   # start under an explicit folder id
-oo dav upload 659 ./historico.xlsx    # multipart upload, upsert by stem|ext
+oo dav upload 659 ./historico.xlsx    # multipart upload, upsert by stem (legacy↔OOXML ext aware)
 oo dav upload 659 ./a.xlsx ./b.pdf --replace=false  # fail on name conflict instead of replacing
 oo dav move 659 22881 22882           # DEST_FOLDER_ID FILE_ID…
 oo dav move 659 22881 --folders 670   # move folders along with files