v3.16.0: test scripts move to a server-side store

The scripts lived in the browser's localStorage although only the local
server can run them.  The new test_scripts.py store
(~/.pysim-simple-server/test_scripts.json, --test-scripts PATH) sits next to
the card presets: stable uuids and timestamps, atomic writes, and every
mutation is validated with the runner's own engine (the submitted shape is
kept - the normaliser's output is not re-normalisable: it fills defaults
like an 'any' PoR check that the check normaliser then rejects), so a stored
script cannot fail at run start for a validation reason.  Import accepts the
old localStorage export as-is (export/import remains the migration path; the
PWA import also takes an exported array).

- /api/test/scripts (GET card-free: path/count + scripts), POST create,
  /update, /delete and /import serve the store; the endpoints answer 503
  without a store and 500 on a write failure, like the preset endpoints.
- /api/test/run takes a stored script_id (resolved when no inline script is
  sent); the PWA sends the edited copy inline plus the id for the report,
  and the run snapshot carries script_id.
- The PWA's Test script pill loads from the store, writes through on edits
  (debounced name, immediate step/new/copy/delete), reports load/save/delete
  failures inline, and exports the portable {name, steps} shape.

Tests: 808 frontend + 701 Python; sw cache simple-v379.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "pysim-simple-server"
-version = "3.15.0"
+version = "3.16.0"
 description = "HTTP REST server wrapping pysim for the SIMple PWA"
 requires-python = ">=3.8"
 # pysim is a git-only dependency installed explicitly by setup.bat/setup.sh.