docs: document Prop 121 disabling outbound IBC, remove dead bridge-out routes and IBC remnants - #51

Well-scoped, accurate documentation update that correctly converts stale "act before the vote" IBC guidance into current-state facts, with good coverage — a repo-wide grep confirms no remaining live IBC remnants outside the touched files, all new anchors resolve, and no stale #ibc-asset-migration-table references survive. No blockers; findings are a stale-date carryover on a line the PR edits, an unsourced date, an anchor worth checking in preview, and a few nits.