Re-home the attractor-explained.html engine-number guards (D-211/D-212/D-213/D-215b) next to the code

[bkrabach]

Filed from microsoft/amplifier-bundle-attractor PR #346 (the P4 slim, attractor-28x). This is a recorded loss of guard strength in attractor that this repo is the right place to restore.

What happened. tests/test_explainer_doc_guard.py in attractor guards the published page docs/attractor-explained.html by reading each engine number out of the engine source and comparing. Four of its five checks read modules/loop-pipeline/amplifier_module_loop_pipeline/:

Check	Number	Engine source (here, @ 1dfc78b)
D-211	max_parallel default = 4	handlers/parallel.py:96
D-212	last_response truncation = 200	handlers/codergen.py:224,244
D-213	summary budgets 600 / 1500 / 3000	fidelity.py:273,285,312
D-215b	apply_transforms() precedes validate_or_raise()	__init__.py:388,391

That package no longer exists in attractor, and its CI cannot read this repo. The checks were re-aimed onto recorded constants (ENGINE_TRUTH, naming file/line/expression/sha), which still catches the page drifting on its own but no longer catches this engine moving underneath the page.

Ask: restore the two-sided form from this side. Either

1. add a guard here that reads these four values from the engine and compares them against the published page fetched from its canonical URL, or
2. take ownership of the page's engine-number claims (the page documents engine internals; it arguably belongs with the engine now).

Whichever lands, attractor's ENGINE_TRUTH block and the four checks that read it should be deleted in the same change — its file states that as its retirement condition.