Add final factorial state to filtering output

[SamDuffield]

Lots of places this will be handy I think

[Copilot]

Pull request overview

This PR updates the factorial offline filtering API to also return the final factorial state when output_factorial=False, and updates tests/docs accordingly.

Changes:

• Extend cuthbert.factorial.filter(..., output_factorial=False) to return (init_state, local_states, final_state).
• Update factorial test suites to unpack the new 3-tuple return and add assertions for final_state.
• Update the factorial README example to reflect the new return shape.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
cuthbert/factorial/filtering.py	Returns final_factorial_state in the non-factorial output path; updates docstring/typing accordingly.
cuthbert/factorial/README.md	Adjusts example usage to unpack the additional final_factorial_state.
tests/cuthbert/factorial/test_kalman.py	Unpacks the additional return value and validates final_state matches reference.
tests/cuthbert/factorial/test_discrete.py	Unpacks the additional return value and validates final_state matches reference.
tests/cuthbert/factorial/test_smc.py	Updates unpacking for the non-factorial output path.
tests/cuthbert/factorial/test_taylor.py	Updates JIT’d call site to unpack the additional return value.

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