fix: make ROSTER.md a dated snapshot and fix grid separators - #12

One of a few QoL improvements from setting up the Q4 audit (#9):

The majority of this diff is just the rosters being re-generated with the formatting fix.

• Adds a 'Last updated' date to ROSTER.md and EMERITUS.md (pulled from data/contributors.yaml) so it's clear what date the 12-month activity counts in ROSTER.md are from.
• Updates generate_roster.dart --check to ignore drift in the 12-month activity columns when sync_activity.yml updates data/activity_summary.json overnight between quarterly reviews, while still checking all members, roles, links, and the 'Last updated' date.
• Fixes a missing • separator at the end of wrapped rows in the Maintainers (ROSTER.md) and Alumni (EMERITUS.md) lists.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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