ADC-693: delete duplicate model and runtime block authoring aliases

[wolf75222]

Scope delivered

This branch now removes two bounded classes of competing Python authoring routes:

• delete pops.physics.Model.to_module(); Model.lower() is the sole explicit advanced projection to an operator-first Module;
• delete MomentModel.check(); MomentModel.build() is the sole construction route;
• delete the Python System.add_block and AmrSystem.add_block wrapper methods;
• prevent __getattr__ from restoring the native add_block ABI as a compatibility fallback in either the assembling or bound lifecycle phase;
• route private native-ModelSpec and compiled-package installation through the existing type-dispatched add_equation seam;
• migrate the remaining direct Python tests to that one private dispatch route and add an AST/passthrough architecture fence.

The public route remains:

Case.block(...) -> resolve -> compile -> bind

Below pops.bind, the private Python wrapper now has one block-installation dispatcher. The C++ System::add_block / AmrSystem::add_block functions remain private native ABI entries consumed by that dispatcher.

History preserved

• explicit merge of current master: 3525467b;
• API cut: 1961f4b1;
• source proof and test migration: d2166323;
• documentation/changelog: 22881e39.

No squash or history rewrite was used.

Validation on the current checkout

• 50 passed: legacy-route architecture, final public API, duplicate-core fences, typed time-factory selectors, ordinary Program factories, and semantic Program identity;
• Ruff passed for every changed Python file;
• docs/check_docs.py: 41 Markdown files verified, with one pre-existing freshness warning for the final specification dependency map;
• git diff --check passed.

The previous smaller head had a fully green GitHub gate; that historical result is not claimed as proof for this refreshed head.

Explicit non-claims

This remains a bounded source slice of ADC-693 and does not close the issue.

• The private add_equation / System / AmrSystem runtime seams still exist and require their own final cutover.
• The C++ native add_block ABI is intentionally retained; deleting or narrowing it needs a coordinated binding/native change.
• No native rebuild, installed-package execution, MPI, OpenMP, GPU, checkpoint/restart, or final-example execution was performed for this new head; those remain CI/integration-gate work.
• PR ADC-693: delete ParaView series compatibility authoring #621 separately removes the ParaView series= compatibility selector.