Add ProjectIndexer (project_lib, QtConcurrent + QFutureWatcher, 6 tests):
runs listFiles/listSymbols on the thread pool, delivers results on the UI
thread via filesReady/symbolsReady, caches per project root, and drops
stale results via a per-root generation counter.
Switchers now open the palette immediately — cache hit is instant, miss
shows a transient 'Indexing…' placeholder — and repopulate when the
background job finishes. The ~0.5-1s ctags stall on Alt+G no longer
blocks the UI thread. Cache invalidated on KateProjectBridge::
projectChanged. Register QList<Symbol> metatype; link Qt6::Concurrent.
Item building factored into buildFileItems/buildSymbolItems (shared by
the cache and async paths).
2bf8d4a3c2