fix(driver-ablegacy): resolve Medium code-review finding (Driver.AbLegacy-004)

DecodeValue for Bit with no bitIndex now reads the full 16-bit word via
GetInt16(0) and tests bit 0 instead of GetInt8(0), which only covered the
low byte and silently misread any bit in positions 8..15. The comment
explains the two decode paths (suffix-present vs suffix-absent).

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

code-reviews/Driver.AbLegacy/findings.md

@@ -117,7 +117,7 @@ reject file numbers on I/O/S, and restrict which file letters may carry a sub-el
 | Severity | Medium |
 | Category | Correctness & logic bugs |
 | Location | `LibplctagLegacyTagRuntime.cs:36-37` |
-| Status | Open |
+| Status | Resolved |
 
 **Description:** `DecodeValue` for `AbLegacyDataType.Bit` with `bitIndex == null`
 returns `_tag.GetInt8(0) != 0`. A bit-file element (`B3:0/0`) is a single bit inside
@@ -132,7 +132,7 @@ but a `Bit`-typed tag configured with an address that has no `/bit` suffix (e.g.
 bit suffix on `Bit`-typed tags (validate in `CreateInstance`/`DiscoverAsync`) or
 decode the full 16-bit word and test bit 0.
 
-**Resolution:** _(open)_
+**Resolution:** Resolved 2026-05-22 — `DecodeValue` for `Bit` with no `bitIndex` now reads the full 16-bit word via `GetInt16(0)` and tests bit 0, avoiding the silent half-word truncation from `GetInt8`.
 
 ### Driver.AbLegacy-005