Add PyRecEst discrete state utilities

[FlorianPfaff]

Summary:

• Add finite-state HMM and IMM discrete-state utilities.
• Export the utilities through the filters registry and public/backend API documentation.
• Add regression coverage for the new discrete-state helpers.

Validation:

• Applied the malformed patch with git apply --recount after git reported incorrect hunk counts.
• Checked for reject files and conflict markers.
• Ran git diff --check on the staged diff.
• Tests were not run per request.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	35.0s
✅ MARKDOWN	markdownlint	2	0	0	0	0.85s
✅ MARKDOWN	markdown-table-formatter	2	0	0	0	0.43s
✅ PYTHON	black	5	2	0	0	3.31s
✅ PYTHON	isort	5	2	0	0	0.5s
✅ REPOSITORY	checkov	yes		no	no	32.23s
✅ REPOSITORY	gitleaks	yes		no	no	14.25s
✅ REPOSITORY	git_diff	yes		no	no	0.05s
✅ REPOSITORY	secretlint	yes		no	no	13.1s
✅ REPOSITORY	syft	yes		no	no	6.07s
✅ REPOSITORY	trivy-sbom	yes		no	no	3.85s
✅ REPOSITORY	trufflehog	yes		no	no	24.2s

Notices

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[github-actions]

Test Results

    10 files  ± 0      10 suites  ±0   2h 31m 25s ⏱️ - 2m 19s
 2 042 tests + 9   2 038 ✅ + 9      4 💤 ±0  0 ❌ ±0 
20 845 runs  +90  17 249 ✅ +90  3 596 💤 ±0  0 ❌ ±0 

Results for commit fbe2c32. ± Comparison against base commit 8cf152a.