Blocking DMV collector: layered wait thresholds + RESOURCE_SEMAPHORE; guard slicer/drill-down vs missing table

[erikdarlingdata]

What

Follow-up to the always-on DMV blocking fallback (#1227/#1230/#1233/#1234). Two things:

1. Layered wait-type thresholds + RESOURCE_SEMAPHORE (collector, both apps)

• Added RESOURCE_SEMAPHORE% to the edge scope (covers both RESOURCE_SEMAPHORE and RESOURCE_SEMAPHORE_QUERY_COMPILE). Confirmed live that these waits do carry blocking_session_id (the memory-grant holders) -- they form real blocker->blocked edges, including the many-to-many case where one memory waiter is reported blocked by every current grant holder.

• Layered minimum-wait floor to cut grid/chain noise by contention class:

  class	floor
  LCK_*	2s
  PAGELATCH_*	0.5s
  PAGEIOLATCH_*	1s
  RESOURCE_SEMAPHORE(_QUERY_COMPILE)	5s

  THREADPOOL deliberately excluded -- no blocking_session_id, and unobservable during real worker starvation.

2. Guard the slicer + flat drill-down against a missing DMV table (regression fix)

The slicer (DatabaseService.QueryPerformance.cs) and flat drill-down (SqlServerDrillDownCollector.Blocking.cs) inline collect.dmv_blocking_snapshots in a single combined CTE/UNION, which fails to compile (Msg 208) on a not-yet-upgraded server -- so the slicer blanked and the drill-down errored. A runtime try/catch can't rescue a single combined batch (unlike the already-guarded pair-row + grid paths, which fetch DMV separately). New BlockingPairRowQuery.DmvSnapshotsTableExistsAsync probe lets both surfaces drop the DMV branch entirely when the table is absent, degrading to BPR-only.

Schema

collect.dmv_blocking_snapshots.lock_mode widened nvarchar(20) -> nvarchar(64) (install/02 + install/06) so the RESOURCE_SEMAPHORE_QUERY_COMPILE tag (32 chars) fits whole. Lite's DuckDB VARCHAR is unbounded (no change).

Testing

• Both apps build clean (0 errors).
• Dashboard 652 / Lite 578 / Installer 80 -- all pass.
• install/56 PARSEONLY-clean.

Not in this PR (held)

• Live apply to SQL2025 (ALTER column + redeploy proc) + Lite rebuild/relaunch -- deferred so as not to perturb the HammerDB validation / evict the running Lite.
• Proper diagnosis of the pre-existing CollectBlockingChainFactsAsync load-error (capture the real exception, not a blind timeout bump).

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