P0.1: restore immediate FAT evidence priority without UI starvation

[masarray]

Root cause

PR #150 intentionally raised FAT evidence dispatch from Background to DataBind so a FALSE→TRUE edge is evaluated and its ON evidence/timestamp is visible before a later FALSE edge. P0 #159 later moved the dispatcher back to Background to avoid UI starvation under a large pending-snapshot queue. The P0 bounded drain (64 snapshots / 4 ms) solved the unbounded-drain problem, but the dispatcher change accidentally regressed #150's immediate edge priority.

Fix

Keep both improvements instead of choosing one:

• a newly scheduled FAT evidence drain is posted at DispatcherPriority.DataBind, preserving immediate TRUE-edge processing;
• if IoTestSessionController reschedules another bounded chunk from inside the active drain, the wrapper detects that active drain and posts the continuation at DispatcherPriority.Background;
• the existing 64 snapshots / 4 ms bounded drain remains unchanged;
• startup evidence batching/hash-chain behavior remains unchanged.

This makes the priority policy burst-aware: first evidence edge is urgent; backlog continuation yields to normal render/input work.

Regression guard

Update the P0 regression contract so it now locks the combined invariant instead of incorrectly requiring Background only:

• bounded 64/4 ms drain still required;
• DataBind required for the first FAT evidence dispatch;
• Background required for continuation dispatch while the drain callback is active;
• increment/decrement guard required around the active drain callback.

Integration safety

Branch is based on current main 6980a2899edad4249ae2e8542d9a9e7f61e6af82, so PR #162 first-run SCL/IP/Excel launcher restoration is already included. Only two files are changed:

• MainWindow.IoTesting.cs
• tests/ARSAS.Tests/IoTestFatP0RegressionTests.cs

No IEC 61850 engine, Clock Sync/SNTP, multi-IED FAT, smart binding, timestamp formatting/evidence precision, clean-FAT semantics, or release metadata is modified.

Do not merge until focused IO FAT checks plus full Windows build/test/package pass on the exact head.