[ajslater]

Problem

picopt -x RAR -c ZIP converted nothing unless ZIP was also passed to -x. Config probed only handlers whose own formats were enabled, so Zip never reached handler_stages, and the repack pass found no available target. Every convert-only route had the same gap: 7Z and TAR/TGZ/TBZ/TXZ to ZIP, CBR/CB7/CBT to CBZ, and image routes like -f BMP -c PNG or -f GIF -c JXL. picopt doctor -x RAR -c ZIP reported FAIL RAR -c ZIP needs -x ZIP.

Fix

• enabled_handler_classes(formats, convert_to) in picopt/config/handlers.py also probes each convert-chain handler of an enabled format whose output is a -c target.
• The doctor uses that same function instead of its own copy of the scope check. Its "needs -x" message can now only appear for the opt-in JXL flags, and it names the missing source format. -f PNG -c JXL --convert-jpeg-to-jxl used to say needs -x JXL, which wouldn't help; it now says JxlFromJpeg needs -x JPEG.
• JxlLossless no longer lists PNG in INPUT_FILE_FORMATS; it was only there to get the handler probed. A PNG run without -c JXL no longer probes the JXL encoder.
• README: the per-directory example drops CBZ from -x. The other examples keep CBZ because they also optimize existing CBZ files.

This doesn't interfere with the #145 stand-ins: a stand-in writes the input's own format, so the existing rule already probes it.

Tests

• test_convert_target_must_be_enabled is now test_convert_target_needs_no_extra_format.
• New tests cover the opt-in "needs -x JPEG" wording and config probing for -x RAR -c ZIP and -f BMP -c PNG.
• Removed the -x ...,ZIP workaround from four tests, so the tar→ZIP and per-directory CBR→CBZ tests now exercise the fix end to end.
• All six new or changed tests fail on the old code.

make fix, make lint and make ty are clean. make test: 439 passed, 6 skipped (PDF render checks).

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