Auto: s7-c4 — deadband / on-change with thresholds

Closes #297

docs/v2/s7.md

@@ -668,6 +668,106 @@ tick rate). Tests can call the internal helpers `S7Driver.GetPartitionCount`
 and `S7Driver.GetPartitionSummary` to inspect the resolved partitioning of
 a live subscription handle.
 
+### Deadband / on-change
+
+Before PR-S7-C4 the subscription poll loop emitted `OnDataChange` whenever
+the freshly-read value differed from the last cached one — a strict
+`!Equals(prev, current)` test. That's correct for booleans and discrete
+state, but for analog tags (Float32 / Float64 / scaled integer set-points)
+it floods the OPC UA subscription queue with insignificant noise: the last
+counts of an ADC's least-significant-bit jitter, sub-percent setpoint drift,
+sensor-grade flutter on a flow rate. PR-S7-C4 lets the operator configure
+**per-tag deadband thresholds** so the driver suppresses uninteresting
+publishes at source, before they cross the OPC UA boundary.
+
+Two knobs, both optional, both per-tag:
+
+- `DeadbandAbsolute` (`double?`) — minimum value change in raw units.
+  Suppress when `|new - prev| < DeadbandAbsolute`.
+- `DeadbandPercent` (`double?`, 0..100) — minimum value change as a
+  percentage of the previous published value. Suppress when
+  `|new - prev| < |prev| * DeadbandPercent / 100`.
+
+When both knobs are set the filters are **OR'd** — the value publishes if
+**either** threshold says publish. This matches Kepware's documented
+"either threshold triggers" semantics and mirrors the AbLegacy driver's
+shipped behaviour for cross-driver consistency.
+
+#### JSON config example
+
+```json
+{
+  "Host": "10.0.0.50",
+  "Tags": [
+    { "Name": "BoilerPressure", "Address": "DB1.DBD0", "DataType": "Float32",
+      "DeadbandAbsolute": 0.5 },
+
+    { "Name": "FlowRate", "Address": "DB1.DBD4", "DataType": "Float32",
+      "DeadbandPercent": 1.0 },
+
+    { "Name": "Temperature", "Address": "DB1.DBD8", "DataType": "Float32",
+      "DeadbandAbsolute": 0.1, "DeadbandPercent": 0.5 }
+  ]
+}
+```
+
+`BoilerPressure` only republishes after a 0.5-bar change; `FlowRate` only
+when the rate moves by more than 1% of its last published value;
+`Temperature` whenever **either** `0.1 °C absolute` **or** `0.5% of last`
+is satisfied.
+
+#### Edge cases
+
+- **First sample.** `PollOnceAsync` gates `forceRaise` and the
+  no-prior-value case ahead of the deadband filter — the first sample for
+  a tag always publishes (otherwise an OPC UA subscription would never see
+  an initial-data push).
+- **Status-code change.** Any transition in the OPC UA `StatusCode` channel
+  (`Bad → Good`, `Good → Bad`, etc.) bypasses deadband and publishes,
+  because quality is a semantically different signal from value.
+- **Non-numeric types.** `String` / `WString` / `Char` / `WChar` /
+  `DateTime` / byte-array tags ignore deadband entirely and keep the
+  legacy `!Equals` semantics. Configuring `DeadbandAbsolute` on a
+  `String` tag is harmless — the filter just doesn't engage.
+- **`NaN` samples.** If either `prev` or `current` is `NaN`, the filter
+  publishes. NaN never equals NaN; treating it as "changed" surfaces the
+  degenerate float to the client rather than hiding it.
+- **`±Infinity` samples.** Same rationale as NaN — degenerate values are
+  always published, never deadbanded.
+- **Sign flip.** A tag swinging `+10 → -10` produces `|delta|=20`; the
+  deadband math operates on the **absolute** delta so a sign flip with
+  `DeadbandAbsolute=1` always publishes. This is the right answer for
+  bidirectional set-points (positive / negative torque, valve-direction
+  flags encoded as signed scalars).
+- **Near-zero baseline (`|prev| < 1e-6`).** A percent threshold against a
+  zero or near-zero baseline diverges (any tiny change is "infinity
+  percent"), so the driver falls back to absolute when `|prev| < 1e-6`:
+  - If `DeadbandAbsolute` is also configured, that threshold takes over.
+  - If only `DeadbandPercent` is set (no absolute fallback), the sample
+    publishes — there's no usable threshold and silently dropping changes
+    against a near-zero baseline would mask a genuine signal.
+
+  The `1e-6` cutoff is a deliberately conservative floor: floats below
+  `~1e-7` are already in denormal-precision territory; anything above
+  `~1e-6` carries enough magnitude that `|prev| * pct / 100` produces a
+  meaningful threshold.
+
+#### Implementation notes
+
+- The filter is the pure-function helper `S7Driver.ShouldPublish(tag,
+  prev, current)`. It's exposed at `internal` scope so unit tests can
+  drive every decision branch (NaN, ±Inf, sign flip, near-zero baseline,
+  both-set OR semantics) without spinning up a partition or poll loop.
+- `LastValues` continues to cache the **last published** snapshot, not
+  the last polled one. After a deadband suppression the next sample
+  compares against the cached (previously published) value, so a slow
+  drift that never crosses the threshold in any single tick still gets
+  caught the moment cumulative drift exceeds the threshold.
+- Deadband is a **publish-time** filter, not a wire-level one — every
+  configured tag is still read every tick, the filter only decides
+  whether to invoke `OnDataChange`. The mailbox / PDU / coalescing path
+  is untouched.
+
 ## TSAP / Connection Type
 
 S7comm runs on top of ISO-on-TCP (RFC 1006), and the COTP connection-request