perf(sidebar): load tree tables before routines to cut database expand latency

[desperadoxhy]

What

Expanding a database in the tree sidebar now loads tables first and fills in procedures and functions in the background.

Why

loadObjects fired three queries through async let: fetchTables, fetchProcedures, fetchFunctions. All three ran on the same driver (one libpq / mariadb connection), and MetadataConnectionPool.runSerially chains same-connection work into strict order, so the async let parallelism was lost. The tree list only appeared after all three round-trips finished in sequence, and most schemas have no routines at all.

Change

• Split DatabaseTreeMetadataService.objects into tablesState and routinesState, each with its own OnceTask dedup.
• loadTables runs only fetchTables. The tree renders as soon as it returns, one round-trip.
• loadRoutines runs procedures and functions on a separate MetadataConnectionPool connection with workload: .bulk, physically isolated from the tables query so they no longer block it.
• objectsContent keys off tablesLoadState; routines load via a .task after tables render and append when ready. MetadataLoadState.isSettled (added) keeps the empty-state from spinning forever if routines fail.
• refreshObjects, handleDisconnect, and resetPending updated for the two split dictionaries.

Verification

• xcodebuild build (Debug, Xcode 27 beta): BUILD SUCCEEDED.
• SchemaServiceTests: 3/3 pass, no regression.
• MetadataLoadStateTests (new): 3/3 pass, covers isSettled.

No unit test for DatabaseTreeMetadataService.loadTables/loadRoutines itself: the service reads DatabaseManager.shared and MetadataConnectionPool.shared singletons and cannot take an injected driver (unlike SchemaService, which the existing tests cover). Decoupling that is a separate refactor.

Notes

Built with Xcode 27 beta (DEVELOPER_DIR=/Applications/Xcode-beta.app). Worth a second pass on a stable Xcode before merge.

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