Convert both directions, and read sedona home from system.properties

Adds the reverse conversion (sab -> sax) and makes the tools usable without
remembering paths on the command line.

- sax.py: decoder ported from OfflineApp.decodeAppBinary + encodeAppXml.
  Rebuilds the tree from the firstChild/nextSibling chain and writes XML the
  way XWriter does. Reproduces java.lang.Float.toString, and rounds manifest
  defaults to f4 before comparing them - otherwise props equal to their
  default (0.1) get written where sedonac omits them.
- sab2sax.py: CLI dispatching on the input extension, like sedonac. sab.py
  still runs and hands off to it.
- config.py: sedona.home lives in system.properties. If it is missing or has
  no manifests/, a folder chooser opens and the answer is written back. File
  choosers for the input, save dialogs for the output; the default output name
  carries a timestamp so a run never overwrites the previous one.
- verify.py: checks both directions plus a sab -> sax -> sab round trip, and
  points at testprogram/ where the test files actually live.

All three checks pass byte-identical against the checked-in sedonac output.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,4 @@
+__pycache__/
+*.pyc
+*-sab2sax-*.sab
+*-sab2sax-*.sax