fix(parallel): align stack word max index bound

[mk0walsk]

• ## Describe your changes

This PR fixes a boundary-contract mismatch in parallel stack word access.

• Updated ReplayProcessor::get_word and ReplayProcessor::set_word to allow start_idx == MIN_STACK_DEPTH - WORD_SIZE (i.e. max valid index 12), matching StackInterface and fast-path behavior.
• Added regression test stack_set_word_allows_max_start_idx to ensure max-index writes/reads remain valid.
• Related: Apply code quality fixes and tests #2802

Checklist before requesting a review

• Repo forked and branch created from next according to naming convention.
• Commit messages and codestyle follow conventions.
• Commits are signed.
• Relevant issues are linked in the PR description.
• Tests added for new functionality.
• Documentation/comments updated according to changes.
• Updated CHANGELOG.md.

[github-actions]

Automated check (CONTRIBUTING.md)

Findings:

• Link an issue in the PR body (e.g., "Fixes Implement SHA256 in Miden Assembly #123").
• Add a short Rationale explaining why the change is needed.

Recommendations:

• Consider adding a Test plan or clear review steps.

Next steps:

• Link a relevant issue (e.g., "Fixes Implement SHA256 in Miden Assembly #123") and ensure it is assigned to you.
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