fix(s7): connect backoff, worst-state write health, Volatile health, array writes (#602, #610)

STAB-21 ConnectionBackoff in EnsureConnectedAsync; STAB-22 write-pass accumulator with a
write-side Faulted that a clean read does not erase; _health on the Volatile idiom;
S7-016 contiguous array write (Bool arrays stay refused).

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

docs/Uns.md

@@ -314,7 +314,7 @@ Combined with historization (values are arrays — history of the whole array sn
 
 ### Out of scope (named deferrals)
 
-- **Array writes** (inbound client→device write of an array value) — tagged for a follow-up phase.
+- **Array writes** (inbound client→device write of an array value) — tagged for a follow-up phase. The S7 driver's array write path exists (`Byte`/`Int16`/`UInt16`/`Int32`/`UInt32`/`Float32`, not `Bool`; see [S7.md](drivers/S7.md)) but the applier still forces array nodes read-only (#610 item 1).
 - **Multi-dimensional arrays** (`ValueRank > 1`) — not supported; all arrays are 1-D.
 - **Array historization** — a historized array tag materialises with the correct `Historizing` flag, but the historian backend treats the value as an opaque blob; per-element history is out of scope. (Continuous historization is numeric-analog only — array / non-numeric values are not recorded.)
 

docs/drivers/S7.md

@@ -195,6 +195,21 @@ Portal under *Protection & Security* for the CPU.
 | PUT/GET denied | `BadNotSupported` | `Faulted` (config alert) |
 | CPU / hardware fault | `BadDeviceFailure` | `Degraded` |
 | Socket / timeout | `BadCommunicationError` | `Degraded` |
+| Reconnect suppressed (backoff window) | `BadCommunicationError` | `Degraded` |
+| Array write of the wrong shape / length / element type | `BadTypeMismatch` | unchanged |
+| Bool / wide-type array write | `BadNotSupported` | unchanged |
+
+**Health is decided once per pass, worst state wins.** A read pass and a write batch each
+accumulate their outcomes and write health once (`Healthy < Degraded < Faulted`), so a PUT/GET-denied
+`Faulted` item is not erased by a later `Degraded` or successful sibling. The write-side `Faulted` is
+**sticky against reads**: a clean read pass (reads often work against a write-protected CPU) leaves
+`Faulted` in place and only advances `LastSuccessfulRead`; the next write that succeeds clears it. A
+transient `Degraded` write failure is still cleared by a clean read, as before.
+
+**Connect backoff.** A dead PLC costs one connect attempt per backoff window (1 s doubling to a 30 s
+cap), not one per read / poll tick / probe tick: inside the window `EnsureConnectedAsync` fails fast
+(`BadCommunicationError`, `Degraded`) without a connect. A successful connect clears the window, so a
+fresh drop after a healthy period reconnects immediately; worst-case recovery lag is the 30 s cap.
 
 ## Testing
 
@@ -228,7 +243,18 @@ normal dev (the S7 sim is on the shared Docker host but currently offline).
 
 **Live-verify** — integration-fixture-gated (not Mac-verifiable without the S7 sim up).
 
-**Deferrals** — array *writes*, multi-dimensional arrays, per-element historization,
+**Array writes (S7-016)** — a writable `Byte` / `Int16` / `UInt16` / `Int32` / `UInt32` / `Float32` array
+is written as ONE contiguous `WriteBytesAsync` of `count x element-bytes` (the inverse of the read
+decode), and the value must be an array of exactly `arrayLength` elements (else `BadTypeMismatch`, no
+health impact). **Writable `Bool` arrays are refused at init** (`NotSupportedException`): the array
+read is byte-granular, so a matching write would overwrite the 7 neighbouring bits of every byte it
+touches, and a safe bit-packed write needs read-modify-write. Mark Bool arrays `writable: false`.
+Note the *server* still presents every array node read-only to OPC UA clients ([Uns.md §Array
+tags](../Uns.md#array-tags-1-d), #610 item 1) — the driver path is reachable today through the Client
+CLI / driver-level callers and is proven by the unit suite plus the snap7 fixture test
+(`S7_1500ArrayWriteTests`); the AdminUI tag editor rejects wide-type and writable-Bool arrays at author time.
+
+**Deferrals** — server-side exposure of array writes, multi-dimensional arrays, per-element historization,
 wide-type array elements (`Int64`, `UInt64`, `Float64`, `String`, `DateTime`), and
 Timer/Counter array elements.