Apply the stricter URL rules only to links a change introduces

Whole-document validation runs when a room opens, restores, rebuilds and
checks each batch, so refusing hidden-character and protocol-relative
links there would lock people out of documents stored before the rule.
The rule now lives in assertIntroducedUrls, which judges only URLs absent
from the previous version: human batches, Planner and MCP edits, and new
documents. Stored documents keep opening, syncing and taking edits.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>