EGI: stabilize mffpy-backed MFF reader

[PragnyaKhandelwal]

Reference issue (if any)

• Follow-up to PR MRG, ENH: Refactor EGIMFF reader to use mffpy (Supersedes #12981) #13684.

What does this implement/fix?

• Ports the EGI MFF reader refactor onto a clean main-based branch.
• Stabilizes the mffpy backend integration.
• Restores compatibility helpers needed by current main.
• Adds/updates EGI MFF test coverage.

Additional information

• This is Week 1 GSoC work building on the earlier prototype.
• I validated the targeted EGI tests locally: 1 passed, 23 skipped.

AI assistance

• I used GitHub Copilot to help draft and review the PR text; I implemented and tested the code changes manually.

[PragnyaKhandelwal]

Hey @scott-huberty and @drammock! Here is the fresh branch for Phase 1 (superseding #13684).
I've ported over the core helper functions we discussed. I am intentionally keeping this in Draft mode because the CI (specifically Vulture) is currently flagging those new helpers as dead code. I haven't fully wired them into the RawMff runtime execution path yet.
My primary goal for this week before our sync is to complete that wiring and decommission the legacy paths to get the CI green. Just wanted to get the core logic up here so we have a shared workspace!

[scott-huberty]

Thanks @PragnyaKhandelwal ! Can you please open a meta-issue on this repo that is titled something like "GSoC 2026: Use mffpy for EGI Reader"? Then you can include one checkbox for each milestone. This can mirror the proposal plan you submitted, but does not need to be as detailed, it can be something like:

• Task 1 #[PR NUMBER]
• Task 2

This will help us keep track of the project's progress. Does that work for you?