Move MIS sampler into backend implementation

[jsenellart]

Motivation

• Rework the previously added MIS implementation so it lives as a simulator backend instead of an algorithm, matching the intended design where MIS is a sampling backend.
• Avoid bundling verification helpers in Perceval (verification to remain external) and rely on an existing strong simulator for exact probabilities.
• Provide a reproducible MCMC-style sampler with configurable burn-in and RNG seeding.

Description

• Remove the algorithm-level file perceval/algorithm/mis.py and stop exporting MIS from the algorithm package.
• Add a new backend implementation perceval/backends/_mis.py implementing MISBackend that builds a distinguishable-photon proposal from |U|^2, applies MIS acceptance using exact probabilities from the SLOS backend, and supports burn_in and seed parameters.
• Register the backend by adding MISBackend to BACKEND_LIST and exposing it under the name "MIS" in perceval/backends/__init__.py so it can be created via BackendFactory.get_backend("MIS", ...) or Processor("MIS", ...).
• Ensure MISBackend delegates target probability evaluation to the existing SLOSBackend rather than embedding verification code.

Testing

• No automated tests were added or executed for this change.
• Basic repository inspection and file creation were performed (files added/removed and BACKEND_LIST updated) but no unit or integration tests were run.
• Manual smoke-checks (listing and viewing modified files) were performed in the development environment and succeeded.
• Recommend adding unit tests for MISBackend.samples/sample and integration tests exercising Processor("MIS") in a follow-up PR.

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Codex Task