fix: harden cell fan-out placement routing (trail #794 findings)

Two low-severity review findings on the mirror fan-out:

• groupReposByCell identified the home placement by string-matching p.Cell == r.Cell to decide which placement inherits the top-level cluster slug. Switch to the contract-guaranteed RepoPlacement.Mirror flag (home = false), so the precise slug->catalog join still works if the index ever omits the top-level Cell alongside the placement array. (finding 019f40a4-e98)

• resolveCellBaseURLs' last-resort jurisdiction-default fallback can bind a group to a non-default cell within its jurisdiction, a silent mirror miss. Add a debug log when that fallback fires so the miss is diagnosable. (finding 019f40a4-fce)

Adds TestGroupReposByCell_PlacementSlugFromMirrorFlag covering an index entry with an empty top-level Cell whose home is identified by Mirror.