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

Nobody has claimed this yet.

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.

Start by reading tests/test_explainer_doc_guard.py and the cited engine paths: handlers/parallel.py, handlers/codergen.py, fidelity.py, and __init__.py. Compare the two proposed ownership approaches and trace the existing ENGINE_TRUTH block and four checks. Done means the four engine-number claims are guarded from the correct source and the obsolete attractor checks are removed.

Written by the indexing model from the issue text.