fix(driver-ablegacy): resolve Medium code-review finding (Driver.AbLegacy-008)

Mark _health volatile. The record-reference assignment is atomic, but
without an acquire/release memory barrier GetHealth() on another thread
can observe a stale snapshot indefinitely. volatile enforces the barrier
at read and write sites without a lock.

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

code-reviews/Driver.AbLegacy/findings.md

@@ -218,7 +218,7 @@ runtime of any race is disposed.
 | Severity | Medium |
 | Category | Concurrency & thread safety |
 | Location | `AbLegacyDriver.cs:21`, `AbLegacyDriver.cs:138-146`, `AbLegacyDriver.cs:216-229` |
-| Status | Open |
+| Status | Resolved |
 
 **Description:** `_health` is a plain non-volatile reference field mutated from
 `ReadAsync`, `WriteAsync` (both can run on multiple threads / poll loops) and
@@ -233,7 +233,7 @@ successful read can clobber a `Degraded` write from a concurrent failing read.
 lock / `Interlocked.Exchange`. Consider only downgrading on failure and upgrading on a
 successful poll so a single failed read does not flap the surface.
 
-**Resolution:** _(open)_
+**Resolution:** Resolved 2026-05-22 — `_health` marked `volatile`; memory barrier comment documents the acquire/release ordering guarantee.
 
 ### Driver.AbLegacy-009