Analyzer::Javascript::RouterMountScanner::PrefixTarget

Where a mount's prefix lands in CodeLocator, and which local variable (if any) should remember it.

Split out of resolve_and_store_router_prefix so the unplaced-mount pass can ask "would this mount land on a key that is still empty?" before any mount is written. Asking after the first write would make the pass order-dependent: two unplaced mounts onto the same child would let whichever ran first veto the other.