A community based topic aggregation platform built on atproto

feat(lexicon): postv2 + acceptance + removal schemas, fixture walker (task 1) master

Author-owned posts PRD §3: social.coves.community.postv2 (author-repo successor to the deprecated community.post — no author field, repo DID is the author), community.acceptance and community.removal (community-repo, key 'any' with deterministic subject-derived rkeys, open knownValues). New T0 whole-tree fixture walker gates tests/lexicon-test-data on the merge path for the first time; shape subtests pin what open schemas can't (author absence, key type, knownValues-not-enum). Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>