fix(physics): InterpolationManager review findings (L.3.1 Task 1 polish)

Addresses code-quality review findings on commit f43f168:

C-1: Stall detection re-implemented to match retail (acclient lines
353071-353275). Tracks _progressQuantum (sum of step values per window)
+ _distanceAtWindowStart (set at window start). Primary check:
cumulative_progress < MIN_DISTANCE_TO_REACH_POSITION (0.20m absolute).
Secondary check: cumulative_progress / _progressQuantum < 0.30.
Either failing increments fail counter; blip-to-tail at >3 consecutive
fails (already correct).

C-2: Renamed StallFailCountForBlip -> StallFailCountThreshold with
clearer XML doc explaining the > vs >= semantics (blip fires when fail
count EXCEEDS the threshold, i.e. on the 4th consecutive failed window).

I-1: _haveBaselineDistance sentinel prevents first-window false
positive that was triggering spurious fails on every new motion sequence
(old code defaulted _distanceAtWindowStart to 0, making cumulative
progress always negative on frame 5).

I-3: dt <= 0 || NaN guard at AdjustOffset entry prevents NaN
propagation into PhysicsBody.Position.

I-4: Internal field renames for clarity:
  _failFrameCounter        -> _framesSinceLastStallCheck
  _failDistanceLastCheck   -> merged into _distanceAtWindowStart

I-5: Added internal Count property + InternalsVisibleTo (via
AssemblyAttribute in .csproj) so Enqueue_DropsOldestWhenAtCap20
actually verifies cap enforcement. Added assertion that head is the
second-enqueued position after overflow.

3 new tests (AdjustOffset_FirstWindow_DoesNotFalseFail,
AdjustOffset_DtZeroOrNegative_ReturnsZero,
Enqueue_AtCap20_HeadIsSecondOriginal), 16 total. All green.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/AcDream.Core/AcDream.Core.csproj

@@ -11,6 +11,11 @@
     <PackageReference Include="Chorizite.DatReaderWriter" Version="2.1.7" />
     <PackageReference Include="Serilog" Version="4.0.2" />
   </ItemGroup>
+  <ItemGroup>
+    <AssemblyAttribute Include="System.Runtime.CompilerServices.InternalsVisibleToAttribute">
+      <_Parameter1>AcDream.Core.Tests</_Parameter1>
+    </AssemblyAttribute>
+  </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\AcDream.Plugin.Abstractions\AcDream.Plugin.Abstractions.csproj" />
   </ItemGroup>