#4666 Phase B — TelehealthComposite 4+2+2 + rebuild subcommand

[jeremydmiller]

Phase B of #4666. Wires the
composite projection topology and the rebuild subcommand so the harness can
actually exercise the daemon's projection paths under load — the regression
bed for the #4667 race fixes.

What ships

Bit	Notes
Stage-2 projections (Domain/Stage2Projections.cs)	Lifted from src/DaemonTests/TeleHealth/: AppointmentDetailsProjection (multi-stream + enrichment), BoardSummaryProjection (multi-stream + ReferencePeerView<Board>), AppointmentMetricsProjection (custom IProjection).
Stage-3 projections (Domain/Stage3Projections.cs) — NEW	ProviderUtilizationProjection (per-provider rollup keyed by ProviderId, reads stage-2 Updated<AppointmentDetails>, enriches with Provider) + TenantDailyRollupProjection (per-day rollup keyed by ISO date string; under conjoined tenancy the date-only key naturally scopes per-tenant, exercising cross-stage chaining + per-tenant aggregation).
CompositeProjection wiring (Program.cs)	opts.Projections.CompositeProjectionFor("TelehealthComposite", ...) with the issue's 4+2+2 stage layout.
SourceGen analyzer dependency (csproj)	Adds JasperFx.Events.SourceGenerator as Analyzer (matches every Marten test project shape). Without it the rebuild fails with No source-generated dispatcher found for SingleStreamProjection<Board, Guid>.
rebuild CLI subcommand	marten-scaletest rebuild [--projection TelehealthComposite] [--shard-timeout-seconds 600]. Builds the daemon, calls RebuildProjectionAsync, prints a Spectre summary. Returns false on any exception so a stress chain can stop early.

Snapshot init guard

AppointmentDetailsProjection.Evolve got a snapshot ??= new ...(id) at the
top: enrichment-added References<T> synthetic events can arrive before the
snapshot-creating Updated<Appointment>, which made the lifted
snapshot!.X = ... pattern NRE on rebuild (smoke caught this immediately —
~16GB memory blow-up before I killed the process, root cause was the
unguarded NRE adding to the daemon's exception/retry queue).

Smoke (local Postgres on docker-compose's localhost:5432)

Step	Result
Build	clean
seed --tenants 2 --events-per-tenant 500	1,136 events / 150 streams, 0.4s
rebuild --projection TelehealthComposite	clean, 0.5s (~2,247 events/sec)

Projection row counts after rebuild (direct psql):

Table	Rows
mt_doc_appointment	100
mt_doc_board	6
mt_doc_providershift	36
mt_doc_appointmentmetrics	16
mt_doc_appointmentdetails	108
mt_doc_boardsummary	6
mt_doc_providerutilization	93
mt_doc_tenantdailyrollup	2

All 8 projections populated end-to-end through the composite's 3 stages.

Phase C / D follow-ups

• Phase C (separate PR): validate subcommand (single-shard baseline diff) + JSON metrics sink + stress chain.
• Phase D: actually run stress at scale against the dev box; if crash-free, close #4667.

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