refactor(basket-compare): rework into a tested, low-noise basket diff

[NavidSassan]

What

Reworks tools/basket-compare from a deepdiff-based, change-only script into a
single generic diff engine, and adds golden-file unit tests.

Why

When a new plugin release ships a regenerated Director basket, importing it
overwrites every object it contains - silently clobbering any manual adjustment
an admin made. basket-compare exists to surface those before the import.

The previous version only printed changed objects and silently dropped
additions and removals (and its add/remove detection was broken by a leaked
loop variable). As a result it missed real cases - e.g. a datafield that an
upcoming import would delete never showed up.

Changes

• Correctness: reports objects only in the left basket, only in the right,
  and changed in both. By default only changed objects are shown (an import
  never touches an object it does not contain); --all adds the Only in ...
  tables.
• Simplification: replaces the ten per-class copy-paste compare_* /
  get_*_diff functions and the deepdiff dependency with one generic engine
  over a normalised {uuid: {field: value}} shape.
• Readability / less noise:
  • a whole added/removed sub-object (a service, a command argument, a field
    binding) collapses into a single row instead of one row per attribute;
  • field-binding rows name the datafield by its varname plus a truncated,
    dash-preserving uuid (still greppable against the basket);
  • list-valued fields diff element by element; over-long values are summarised;
  • suppresses known Director export noise - the is_required flag (which does
    not round-trip) and values stored as a number on one side and the same
    digits as a string on the other.
• Tests: new tools/unit-test/basket-compare/ golden-file tests (fixtures +
  run), robust to the trailing-whitespace / end-of-file pre-commit hooks.
• Docs: TOOLS.md basket-compare section updated.

Impact

On a real upgrade export the output dropped from ~1700 to ~440 lines while now
catching object additions/removals the old version missed.

Notes

• tools/ scripts are not auto-discovered by the CI test harness
  (run-unit-tests covers check-plugins/ only); the new test runs via
  tools/unit-test/basket-compare/run. Wiring tools tests into CI is a possible
  follow-up.
• No CHANGELOG entry: basket-compare is an internal maintainer tool, not part
  of the shipped, admin-facing plugin collection.