Fix MEM-RBPF FFBSi tuple record handling

[FlorianPfaff]

Summary

• fix MEMRBPFFFBSiSmoother legacy tuple-record handling so documented 5-tuples no longer omit the required particle weights
• infer uniform weights for legacy 5-tuples and 6-tuples whose last element is the kwargs mapping
• support explicit (x, P, theta, axis, axis_covariance, weights, kwargs) tuple records without breaking the existing mapping and object paths
• add regression tests for legacy uniform-weight tuple records and explicit weights plus kwargs

Rationale

_as_record advertised tuple records of length 5 or 6, but a 5-tuple was forwarded directly into MEMRBPFForwardRecord without the required weights argument. That made the legacy tuple compatibility path unusable. A 6-tuple was also ambiguous because it was always treated as (five fields, kwargs), leaving no way to provide explicit weights together with keyword metadata.

Validation

• Verified the branch is based on current main via connector compare: 2 commits ahead, 0 behind.
• Reviewed the connector diff: 2 files changed (src/pyrecest/smoothers/mem_rbpf_ffbsi_smoother.py, tests/smoothers/test_mem_rbpf_ffbsi_smoother.py).
• Direct local pytest execution was not possible from this environment because the sandbox cannot clone/resolve GitHub; PR CI should run the focused smoother tests and full matrix.

[github-actions]

✅⚠️MegaLinter analysis: Success with warnings

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	54.71s
✅ JSON	prettier	7	0	0	0	0.87s
✅ JSON	v8r	7		0	0	5.07s
⚠️ MARKDOWN	markdownlint	66	0	2	0	1.99s
✅ MARKDOWN	markdown-table-formatter	66	1	0	0	0.64s
✅ PYTHON	black	758	2	0	0	38.77s
✅ PYTHON	isort	758	2	0	0	2.23s
✅ REPOSITORY	checkov	yes		no	no	39.64s
✅ REPOSITORY	gitleaks	yes		no	no	18.2s
✅ REPOSITORY	git_diff	yes		no	no	0.06s
✅ REPOSITORY	secretlint	yes		no	no	23.1s
✅ REPOSITORY	syft	yes		no	no	2.9s
✅ REPOSITORY	trivy-sbom	yes		no	no	5.97s
✅ REPOSITORY	trufflehog	yes		no	no	24.16s
✅ YAML	prettier	10	0	0	0	0.76s
✅ YAML	v8r	10		0	0	13.86s
✅ YAML	yamllint	10		0	0	0.53s

Detailed Issues

⚠️ MARKDOWN / markdownlint - 2 errors

.github/pull_request_template.md:1 error MD041/first-line-heading/first-line-h1 First line in a file should be a top-level heading [Context: "## Summary"]
docs/backend-support.md:11:174 error MD060/table-column-style Table column style [Table pipe does not align with header for style "aligned"]

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