docs: clarify S7 counter evidence and fixture limits

docs/drivers/S7.md

@@ -198,15 +198,53 @@ Timers are **read-only** this phase; a write attempt returns `BadNotWritable`.
 ### Counter read
 
 **Address**: `C{n}` (e.g. `C0`, `C10`). **DataType must be `Int32`**. The OPC UA node is
-a scalar `Int32` whose value is the current **count** (raw word from the PLC).
+a scalar `Int32` containing the unsigned **raw big-endian word**, widened to `Int32`:
+`(bytes[0] << 8) | bytes[1]`. `S7.Net`'s `Counter.FromByteArray` performs no BCD
+reinterpretation; for example, bytes `01 23` surface as `291`. This describes the
+current decoder, not verified agreement with a PLC's displayed count.
 Counters are **read-only** this phase; a write attempt returns `BadNotWritable`.
 
-> **Known limitation — Counter BCD encoding on S7-300/400**: `S7.Net`'s
-> `Counter.FromByteArray` returns the raw big-endian word without BCD decode.
-> On **classic S7-300/400** the C-area word is BCD-encoded (0–999), so the
-> surfaced value can differ from the actual count on that hardware. S7-1200/1500
-> use IEC/DB counters (plain integers) where the raw word is correct. BCD
-> reinterpretation for legacy C-area is a live-hardware-gated follow-up.
+The S7comm ReadVar **C-area response encoding remains unverified on hardware**.
+PLC instruction representations do not by themselves establish network encoding:
+Siemens documents a binary current value for [L: Load counter](https://docs.tia.siemens.cloud/r/en-us/v21/stl-s7-300-s7-400/stl-mnemonic-s7-300-s7-400/counter-operations-s7-300-s7-400/l-load-counter-s7-300-s7-400)
+and separate `CV` and `CV_BCD` outputs for [S_CD](https://docs.tia.siemens.cloud/r/en-us/v21/lad-s7-300-s7-400/counter-operations-s7-300-s7-400/simatic-counters-s7-300-s7-400/s_cd-assign-parameters-and-count-down-s7-300-s7-400?contentId=reR_xapFX_Ike2opHIvfzg).
+
+IEC counter values stored in DB memory use the **DB address and declared integer
+type**, a different read path from `C{n}`. CPU family alone does not select the
+counter kind or prove its wire representation. In particular, Siemens also lists
+legacy SIMATIC counters for S7-1500 in its [instruction comparison (pages 23–24)](https://cache.industry.siemens.com/dl/files/777/109986777/att_1322640/v1/s7_1500_compare_table_en.pdf?download=true).
+The driver has no counter encoding selector; hardware-faithful counter support
+requires independent raw-response evidence before any interpretation change.
+
+### Counter evidence follow-up (#616)
+
+The existing `S7_1500SpanReadTests` exercises the python-snap7 profile's synthetic
+raw words (`C0 = 4711`, `C7 = 31337`). These are outside the classic SIMATIC count
+range `0..999` documented by S_CD. These expectations describe raw-word handling;
+they do not establish classic counter count semantics. The recorded device-0 run cannot prove nonzero
+numbering. The nonzero test is environment-gated, but its options remain
+`CpuType = S71500`, rack `0`, slot `0`: setting `S7_TIMER_NUMBER_ADDRESSING=1`
+does not configure a classic CPU or make the profile a classic counter gate.
+
+Before claiming hardware validation or closing #616:
+
+- Prepare a separate target-specific semantic gate with the actual CPU type,
+  rack, slot and TSAP/connection settings. Keep the synthetic raw-word profile
+  as its own evidence of the existing decoder contract.
+- Record CPU model/order number, firmware, endpoint, connection settings and
+  driver/S7netplus plus simulator versions where applicable.
+- Observe independent PLC-side current counts and binary/BCD reference outputs
+  for valid classic counts such as `123`, a second distinguishing count (`42`
+  or `999`), and zero. Record the instruction/counter kind and DB reference
+  addresses; do not derive reference values from the driver's output.
+- Capture the actual two-byte S7comm C-area responses alongside those references.
+  Do not infer response bytes from a simulator seed or expected BCD value.
+- Verify several distinct nonzero timer/counter numbers against their independent
+  PLC values and captured requests/responses, and verify the read-only contract.
+
+The raw-byte/count and nonzero-numbering gates remain open. Password/protection
+support inherited from #303/#632 is a separate follow-up, not evidence supplied
+by these counter checks.
 
 ### Deferrals (not yet implemented)