Strands: index the registry by child worldline #902

Raised by CodeRabbit on PR #890 (#890 (comment)). StrandRegistry::find_by_child_worldline scans every strand. The occurrence-inheritance walk calls it once per ancestry level, and once on every occurrence admission without a local binding, so admission costs O(strands × depth). The trusted host caps strands at 64, so this is bounded today.

Now that StrandRegistry::insert enforces one relation per child worldline (ChildWorldlineAlreadyRelated, #890), the relation is a function. Keep a BTreeMap<WorldlineId, StrandId> index, maintained in insert and remove, and back find_by_child_worldline with it, so the walk is O(depth · log n) and stays deterministic.

Acceptance: the index stays consistent across insert, remove and recovery; existing strand tests pass.