Tech collective: clean up stale votes after member removal (F-91265)

[n13]

Summary

Adds the substantive fix from the v12 suggestions (authored by illuzen) that closes audit finding F-91265 (a removed member's vote stays in an ongoing poll's tally) in the surviving tech-collective lane.

• Cherry-picks allow removing votes from poll if member removed (illuzen): adds a permissionless remove_vote_for_non_member(who, poll) extrinsic to pallet-ranked-collective (call_index(7)) that clears a non-member's stale vote from an Ongoing poll and corrects the tally. Guarded to only act on accounts that are not current members (StillMember), that actually voted (NotVoted), on polls still ongoing (NotPolling). Fee waived (cleanup op). Ships 6 unit tests.
• Bumps runtime spec_version 133 → 134 (new extrinsic).
• Documents the call in docs/TECH_REFERENDA_FEATURES.md.

The second suggested commit was intentionally dropped

The other suggested commit (fix zero-denominator issue in pallets/conviction-voting, F-91141) does not apply here: pallet-conviction-voting was removed entirely in #604, so the bug is moot. The surviving tech-lane tally already guards the zero-denominator case:

Perbill::from_rational(self.ayes, 1.max(self.ayes + self.nays))

Test plan

• cargo test -p pallet-ranked-collective → 21 passed (incl. 6 new: remove_vote_for_non_member_works, ..._removes_nay_vote, ..._fails_if_not_voted, ..._fails_on_completed_poll, ..._fails_if_still_member, remove_member_cleanup_works)
• cargo check -p quantus-runtime → clean; extrinsic exposed via TechCollective

Notes

• New extrinsic at a fresh call_index(7) is additive/backward-compatible → transaction_version left unchanged.
• Weight reuses WeightInfo::vote() (O(1), comparable work) → no new benchmark required.

[n13]

The edge case here is

What if a poll is ongoing, and a tech collective member is removed while the poll is ongoing.

In our current setting only 1 poll can be active at a time

Therefore you can't remove a member while the poll is ongoing (needs a 2nd poll)

also I don't see why this is a better solution - the member was part of the tech collective when the voting began, then voted...

Removing them if they're no longer eligible is a choice, but keeping them is equally valid...

I don't think we should do this

Claude opened a PR for it already, but I think it just adds some complications that then also have other impacts potentially, less code is more