Replace the NotSupportedException at AdsTwinCATClient.WriteValueAsync
for bit-indexed BOOL writes with a read-modify-write path:
1. Strip the trailing .N selector from the symbol path.
2. Read the parent as UDINT.
3. Set or clear bit N via the standard mask.
4. Write the parent back.
Concurrent bit writers against the same parent serialise through a
per-parent SemaphoreSlim cached in a ConcurrentDictionary (never
removed — bounded by writable-bit-tag cardinality). Mirrors the AbCip /
Modbus / FOCAS bit-RMW pattern shipped in #181 pass 1.
The path-stripping (TryGetParentSymbolPath) and mask helper (ApplyBit)
are exposed as internal statics so tests can pin the pure logic without
needing a real ADS target. The FakeTwinCATClient mirrors the same RMW
semantics so driver-level round-trip tests assert the parent-word state.
Closes#307
fcf89618cd