feat(physics #145): Slice 1 — Position/Frame types + LandDefs.GetBlockOffset (0x0043e630)

Introduces the two value types (Frame, Position) that represent retail's
cell-relative position pair (acclient.h:30647/30658). Types are unused
by consumers yet — zero behavior change. Also ports LandDefs::get_block_offset
(pc:69189, @0x0043e630): world-meter offset between two named landblock ids,
the ONLY cross-cell translation primitive in retail physics. Conformance tests:
same-landblock→Zero, south-neighbour→(0,-192,0) (the exact #145 cascade cell),
east-neighbour→(+192,0,0), diagonal→(+192,+192,0). 4/4 pass; full Core suite
1522 passed / 0 failed. DatFrame alias added to 4 files that had using
DatReaderWriter.Types + using AcDream.Core.Physics in scope simultaneously.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Physics/Position.cs

@@ -0,0 +1,23 @@
+using System.Numerics;
+
+namespace AcDream.Core.Physics;
+
+/// <summary>
+/// Retail <c>Frame</c> (acclient.h:30647). <see cref="Origin"/> is LOCAL to the
+/// owning cell: outdoor → X/Y ∈ [0,192) within the landblock, Z = height;
+/// indoor → the EnvCell-relative placement. <see cref="Orientation"/> replaces
+/// retail's <c>m_fl2gv</c> 3×3 local→global matrix (equivalent rotation).
+/// </summary>
+public readonly record struct Frame(Vector3 Origin, Quaternion Orientation);
+
+/// <summary>
+/// Retail <c>Position</c> (acclient.h:30658): the (which-cell, where-inside) pair.
+/// One type for indoor and outdoor. The full 32-bit cell id (high 16 = landblock
+/// prefix, low 16 = cell index) plus the cell-local <see cref="Frame"/>. Neither
+/// half is ever reconstructed from a streaming center — see #145 design spec.
+/// </summary>
+public readonly record struct Position(uint ObjCellId, Frame Frame)
+{
+    public Position(uint objCellId, Vector3 origin, Quaternion orientation)
+        : this(objCellId, new Frame(origin, orientation)) { }
+}