Auto: ablegacy-8 — per-tag deadband / change filter

Closes #251

docs/Driver.AbLegacy.Cli.md

@@ -95,6 +95,31 @@ PLC-managed — use with caution.
 otopcua-ablegacy-cli subscribe -g ab://192.168.1.20/1,0 -a N7:10 -t Int -i 500
 ```
 
+#### Deadband
+
+PR 8 — per-tag absolute / percent change filter on top of the polled subscription. The driver
+caches the last *published* value per tag and suppresses `OnDataChange` notifications until the
+new sample crosses the configured threshold.
+
+| Flag | Effect |
+|---|---|
+| `--deadband-absolute <value>` | Suppress until `|new - prev| >= value`. |
+| `--deadband-percent <value>` | Suppress until `|new - prev| >= |prev * value / 100|`. `prev == 0` always publishes (avoids div-by-zero). |
+
+Booleans bypass the filter entirely (every transition publishes); strings + status changes
+always publish; first-seen always publishes; both flags set → either passing triggers a
+publish (Kepware-style logical OR).
+
+```powershell
+# Float — drop sub-0.5 jitter from the noisy load-cell address.
+otopcua-ablegacy-cli subscribe -g ab://192.168.1.20/1,0 -a F8:0 -t Float -i 500 `
+    --deadband-absolute 0.5
+
+# Integer — only fire on >= 5% deviation from the last reported value.
+otopcua-ablegacy-cli subscribe -g ab://192.168.1.20/1,0 -a N7:10 -t Int -i 500 `
+    --deadband-percent 5
+```
+
 ## Array reads
 
 PR 7 — one PCCC frame can carry up to ~120 words. Address an array tag with either the

docs/drivers/AbLegacy-Test-Fixture.md

@@ -49,6 +49,12 @@ supplies a `FakeAbLegacyTag`.
 - `AbLegacyHostAndStatusTests` — probe + host-status transitions driven by
   fake-returned statuses
 - `AbLegacyDriverTests` — `IDriver` lifecycle
+- `AbLegacyDeadbandTests` — PR 8 per-tag deadband / change filter:
+  absolute-only suppression sequence `[10.0, 10.5, 11.5, 11.6] -> [10.0, 11.5]`,
+  percent-only suppression with a zero-prev short-circuit, both-set logical-OR
+  semantics (Kepware), Boolean edge-only publish, string change-only publish,
+  status-change always-publish, first-seen always-publish, ReinitializeAsync
+  cache wipe, JSON DTO round-trip.
 
 Capability surfaces whose contract is verified: `IDriver`, `IReadable`,
 `IWritable`, `ITagDiscovery`, `ISubscribable`, `IHostConnectivityProbe`,