fix(driver-focas): resolve Medium code-review finding (Driver.FOCAS-003)

Throw InvalidOperationException at InitializeAsync when a tag's
DeviceHostAddress does not match any entry in the Devices list, naming
both the tag and the unresolved host.  Previously the missing-device
check was guarded by a TryGetValue so a typo silently bypassed
capability-matrix validation and deferred the error to per-read
BadNodeIdUnknown — the opposite of the documented "fail at load" goal.

Also resolves findings 004, 005, and 006 in the same file:
- 004: DiscoverAsync now unconditionally emits ViewOnly for all user
  tags; the Writable config field no longer influences security class
  because the wire backend always returns BadNotWritable.
- 005: All _health reads use Volatile.Read and all writes use
  Volatile.Write so concurrent readers observe a consistent reference
  and read-modify-write sequences capture a stable snapshot.
- 006: EnsureConnectedAsync disposes and nulls any existing
  non-connected client before creating a fresh one, preventing
  ObjectDisposedException loops after a HandleRecycle race or teardown.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

code-reviews/Driver.FOCAS/findings.md

@@ -7,7 +7,7 @@
 | Review date | 2026-05-22 |
 | Commit reviewed | `76d35d1` |
 | Status | Reviewed |
-| Open findings | 10 |
+| Open findings | 9 |
 
 ## Checklist coverage
 
@@ -95,7 +95,7 @@ or op-mode read to be `IsOk` before declaring the capability present.
 | Severity | Medium |
 | Category | Correctness & logic bugs |
 | Location | `FocasDriver.cs:71-79` |
-| Status | Open |
+| Status | Resolved |
 
 **Description:** In `InitializeAsync`, capability-matrix validation only runs when
 `_devices.TryGetValue(tag.DeviceHostAddress, out var device)` succeeds. A tag whose
@@ -110,7 +110,7 @@ that "config errors now fail at load instead of per-read"
 `tag.DeviceHostAddress`, throw an `InvalidOperationException` naming the tag and the
 unresolved device host so the operator fixes the typo at startup.
 
-**Resolution:** _(open)_
+**Resolution:** Resolved 2026-05-22 — `InitializeAsync` now throws `InvalidOperationException` naming the tag and the unresolved device when `_devices` does not contain `tag.DeviceHostAddress`, preventing silent skip-and-defer to per-read `BadNodeIdUnknown`.
 
 ### Driver.FOCAS-004