feat(R2-Q1): verbatim MotionState (gap H2)

Style/Substate/SubstateMod + the two independent MotionList chains with
retail's exact disciplines: modifier PUSH-FRONT stack
(add_modifier_no_check 0x00525ff0; add_modifier 0x00526340 refusing
duplicates AND the current base substate; remove_modifier by node;
clear_modifiers) and action TAIL-APPEND FIFO (add_action 0x005260a0;
remove_action_head 0x00526120 returning the popped motion, 0 when
empty; clear_actions). Deep-copy ctor clones both chains (Q0-pins
A4-#5: re_modify's snapshot is a termination bound, never shared).

11 discipline-table tests. Next: Q2 (CMotionTable + free functions —
the GetObjectSequence dispatcher).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/AcDream.Core/Physics/Motion/MotionState.cs

@@ -0,0 +1,122 @@
+using System.Collections.Generic;
+
+namespace AcDream.Core.Physics.Motion;
+
+/// <summary>
+/// One entry of a <see cref="MotionState"/> chain — retail's
+/// <c>MotionList</c> node (12 bytes: motion + speed_mod + next;
+/// acclient.h, r2-motiontable-decomp.md §14). One struct, two independent
+/// chain disciplines: the modifier chain is a PUSH-FRONT STACK, the action
+/// chain a TAIL-APPEND FIFO.
+/// </summary>
+public sealed class MotionEntry
+{
+    public uint Motion;
+    public float SpeedMod = 1f;
+
+    public MotionEntry(uint motion, float speedMod)
+    {
+        Motion = motion;
+        SpeedMod = speedMod;
+    }
+}
+
+/// <summary>
+/// R2-Q1 — verbatim port of retail's <c>MotionState</c> (acclient.h:31081;
+/// r2-motiontable-decomp.md §13): the motion-table-facing state block —
+/// current style / substate / substate_mod plus the modifier stack and the
+/// action FIFO that <c>CMotionTable::GetObjectSequence</c>,
+/// <c>StopSequenceMotion</c>, <c>re_modify</c>, and
+/// <c>MotionTableManager::AnimationDone</c> read and write.
+/// </summary>
+public sealed class MotionState
+{
+    /// <summary>Current style/stance command word (0 = unset; ctor 0x00525fd0).</summary>
+    public uint Style;
+
+    /// <summary>Current base substate command word (0 = unset).</summary>
+    public uint Substate;
+
+    /// <summary>Speed modifier of the base substate (default 1.0).</summary>
+    public float SubstateMod = 1f;
+
+    private readonly LinkedList<MotionEntry> _modifiers = new(); // modifier_head — push-front stack
+    private readonly LinkedList<MotionEntry> _actions = new();   // action_head/tail — FIFO
+
+    public MotionState()
+    {
+    }
+
+    /// <summary>
+    /// Deep copy (retail copy ctor 0x00526790; Q0-pins A4-#5): both chains
+    /// CLONED — <c>re_modify</c>'s snapshot is a termination bound, never
+    /// shared state.
+    /// </summary>
+    public MotionState(MotionState other)
+    {
+        Style = other.Style;
+        Substate = other.Substate;
+        SubstateMod = other.SubstateMod;
+        foreach (var m in other._modifiers)
+            _modifiers.AddLast(new MotionEntry(m.Motion, m.SpeedMod));
+        foreach (var a in other._actions)
+            _actions.AddLast(new MotionEntry(a.Motion, a.SpeedMod));
+    }
+
+    /// <summary>Modifier chain in retail order (newest first — push-front).</summary>
+    public IEnumerable<MotionEntry> Modifiers => _modifiers;
+
+    /// <summary>Action FIFO in retail order (oldest first).</summary>
+    public IEnumerable<MotionEntry> Actions => _actions;
+
+    /// <summary><c>add_modifier_no_check</c> (0x00525ff0): unconditional
+    /// push-front.</summary>
+    public void AddModifierNoCheck(uint motion, float speedMod)
+        => _modifiers.AddFirst(new MotionEntry(motion, speedMod));
+
+    /// <summary>
+    /// <c>add_modifier</c> (0x00526340): refuse when the motion is already
+    /// a modifier (caller must stop-then-re-add to refresh) or already IS
+    /// the current base substate.
+    /// </summary>
+    public bool AddModifier(uint motion, float speedMod)
+    {
+        for (var n = _modifiers.First; n is not null; n = n.Next)
+            if (n.Value.Motion == motion)
+                return false;
+
+        if (Substate == motion)
+            return false;
+
+        AddModifierNoCheck(motion, speedMod);
+        return true;
+    }
+
+    /// <summary><c>remove_modifier</c> (0x00526040) — by node identity
+    /// (retail's node+predecessor pair collapses in a managed list).</summary>
+    public void RemoveModifier(MotionEntry entry)
+    {
+        _modifiers.Remove(entry);
+    }
+
+    /// <summary><c>clear_modifiers</c> (0x00526070).</summary>
+    public void ClearModifiers() => _modifiers.Clear();
+
+    /// <summary><c>add_action</c> (0x005260a0): tail append.</summary>
+    public void AddAction(uint motion, float speedMod)
+        => _actions.AddLast(new MotionEntry(motion, speedMod));
+
+    /// <summary><c>remove_action_head</c> (0x00526120): pop the FIFO head,
+    /// returning its motion id (0 when empty).</summary>
+    public uint RemoveActionHead()
+    {
+        var head = _actions.First;
+        if (head is null)
+            return 0;
+        _actions.RemoveFirst();
+        return head.Value.Motion;
+    }
+
+    /// <summary><c>clear_actions</c> (0x005260f0).</summary>
+    public void ClearActions() => _actions.Clear();
+}