chore(audit): ScadaBridge C7 — perf re-baseline + CollapseAuditLogToCanonical projection test + index-test fix + dead-cref cleanup (Task 2.5)

Perf re-baseline (HotPathLatencyTests): empirical p95 on Apple M-series Release
build: 4KB DetailsJson slow path ≈14 µs, small-DetailsJson no-redactors ≈2 µs,
true no-op fast path ≈0 µs. Thresholds updated: 200 µs / 30 µs / 5 µs (≈15×
headroom for contested CI runners). Old thresholds (50 µs / 10 µs) were set for
the pre-C3 typed-field path; canonical JSON parse+rewrite is empirically faster.
Adds a third test (Filter_Apply_NoDetailsJson_FastPath) that asserts same-instance
return on the DetailsJson-null + within-cap fast path. Env-var overrides retained.

CollapseAuditLogToCanonicalMigrationTests (new): three MSSQL-gated [SkippableFact]
tests verifying Action/Category/Outcome projection, NULL Actor, DetailsJson codec
round-trip, and all six persisted computed columns (Kind/Status/SourceSiteId/
ExecutionId/ParentExecutionId) for ApiOutbound, InboundAuthFailure, and Failed-
status rows.

AddAuditLogTableMigrationTests: rename CreatesFiveNamedIndexes →
CreatesNineNamedIndexes; expand coverage from 5 original indexes to all 9 named
non-clustered indexes present after CollapseAuditLogToCanonical (adds
IX_AuditLog_Execution, IX_AuditLog_ParentExecution, IX_AuditLog_Node_Occurred,
UX_AuditLog_EventId).

Dead-cref cleanup: zero references to the deleted IAuditPayloadFilter /
DefaultAuditPayloadFilter / SafeDefaultAuditPayloadFilter types remain in any
.cs file (source or test). 26 occurrences across 13 files replaced with correct
references to IAuditRedactor / ScadaBridgeAuditRedactor / SafeDefaultAuditRedactor
or reworded as plain prose.

Residual sweep: no unused transitional code found beyond the acknowledged
"C3 transitional shim" comments on IngestedAtUtc stamping (active code, not dead).

src/ZB.MOM.WW.ScadaBridge.AuditLog/Central/CentralAuditWriter.cs

@@ -82,11 +82,11 @@ public sealed class CentralAuditWriter : ICentralAuditWriter
         _services = services ?? throw new ArgumentNullException(nameof(services));
         _logger = logger ?? throw new ArgumentNullException(nameof(logger));
         // AuditLog-008: never default to null — over-redact instead.
-        // C3 (Task 2.5): the canonical IAuditRedactor replaces the legacy
-        // IAuditPayloadFilter. SafeDefaultAuditRedactor applies HTTP header
-        // redaction with hard-coded sensitive defaults so a composition root
-        // that omits the real redactor still scrubs Authorization / X-Api-Key /
-        // Cookie / Set-Cookie before persistence.
+        // C3 (Task 2.5): wired via the canonical IAuditRedactor seam.
+        // SafeDefaultAuditRedactor applies HTTP header redaction with
+        // hard-coded sensitive defaults so a composition root that omits the
+        // real redactor still scrubs Authorization / X-Api-Key / Cookie /
+        // Set-Cookie before persistence.
         _redactor = redactor ?? SafeDefaultAuditRedactor.Instance;
         _failureCounter = failureCounter ?? new NoOpCentralAuditWriteFailureCounter();
         _nodeIdentity = nodeIdentity;