Give mixed selections something to do

A Drag of different kinds showed an empty wheel, because the catalog only
answers for a single category. Filee's answer is a mixed profile, so add
Zip (one archive, flat names) and Merge (non-PDFs are converted to PDF
first); a multi-file drag of one kind gets Zip too.

README.md

@@ -23,6 +23,9 @@ Everything runs locally — no uploads, no accounts, no analytics.
    Dropping on the dead zone in the middle, pressing Escape, or dropping the
    file anywhere else cancels and lets Finder do its normal thing.
 
+Mixed selections still work: **Zip** packs the selection into one archive and
+**Merge** turns it into one PDF (non-PDFs are converted first).
+
 The wheel also works without dragging: the menu bar item and the Finder Quick
 Action **Convert with Konverter…** open the same options as a menu.
 
@@ -55,7 +58,7 @@ finished, sound, use of external tools, and open at login. Stored in
 | Subtitle | SRT, VTT, TXT |
 | Archive / folder | ZIP, TAR, GZIP |
 
-Plus 38 tools:
+Plus 39 tools:
 
 | Group | Tools |
 | --- | --- |
@@ -66,9 +69,17 @@ Plus 38 tools:
 | Document | To PDF, Statistics |
 | Spreadsheet | To CSV |
 | Subtitle | Shift Time |
-| Archive | Extract, List Contents |
+| Archive | Extract, List Contents, Zip |
 | Any | Info |
 
+### Mixed selections
+
+Dragging files of different kinds still shows a wheel instead of nothing:
+**Zip** packs the whole selection into one archive (flat names, like Finder's
+*Compress*), and **Merge** turns everything into one PDF — photos, documents
+and existing PDFs are converted to PDF first, then combined. A multi-file drag
+of a single kind gets **Zip** in its tools wheel too.
+
 ### Engines and fallbacks
 
 | Layer | Used for | Ships with macOS |
@@ -112,7 +123,7 @@ Requires Xcode (Swift 6 toolchain) and macOS 13+.
 swift build                       # debug binary
 swift build -c release
 
-./.build/debug/Konverter --self-test   # 41 checks over every engine
+./.build/debug/Konverter --self-test   # 46 checks over every engine
 ./.build/debug/Konverter --convert photo.heic --to png
 ./.build/debug/Konverter --convert clip.mov --tool trimVideo \
     --param start=0:02 --param end=0:05