feat: expose opt-in EP updater through declarative optimise

[Jammy2211]

Summary

Expose the existing expectation-propagation updater hierarchy through the declarative FactorGraphModel.optimise() API.
An explicitly supplied updater is passed unchanged to EPOptimiser.
The default remains updater=None, preserving full, undamped updates via SimplerUpdater(delta=1.0).
Export updater classes from the public autofit and autofit.graphical namespaces.
Improve collapse diagnostics and documentation to frame damping as problem-dependent rather than a universal fix.

Closes #1456.

API Changes

FactorGraphModel.optimise() now accepts an optional updater keyword.
ApproxUpdater, DynamicUpdater, FactorUpdater, and SimplerUpdater are now available from autofit.
Omitting updater retains the existing undamped delta=1.0 behaviour; damping remains opt-in.
No migration is required for existing callers.
See full details below.

Test Plan

• python -m pytest test_autofit/ -x -q — 1667 passed, 6 skipped
• Focused declarative and diagnostic tests — 17 passed
• python -m compileall and git diff --check

Full API Changes (for automation & release notes)

Added

• autofit.ApproxUpdater — public base updater interface.
• autofit.DynamicUpdater — public dynamically selected update policy.
• autofit.FactorUpdater — public per-factor update policy.
• autofit.SimplerUpdater — public fixed-delta update policy.
• The same updater symbols are exported from autofit.graphical and autofit.graphical.expectation_propagation.

Changed Signature

• autofit.FactorGraphModel.optimise(optimiser, paths=None, ep_history=None, updater=None, **kwargs) — accepts an optional EP update policy.
• Declarative lowering passes an explicitly supplied updater unchanged to EPOptimiser.

Changed Behaviour

• Explicit updater=... values now control declarative EP message updates.
• Omitted updater continues to use SimplerUpdater(delta=1.0); no damping is enabled by default.

Migration

• Existing calls require no changes.
• Opt in with factor_graph.optimise(optimiser, updater=af.SimplerUpdater(delta=0.5)).

Generated by the PyAutoLabs agent workflow.

[Jammy2211]

Workspace PR: PyAutoLabs/autofit_workspace#136

This draft remains behind the library-first merge gate and should merge only after this PyAutoFit PR.