fix(input): Phase K live-test fixes pt3 — fly→chase round-trip, Shift coexists, run-speed for backward + strafe

Four issues from the K-fix2 launch (2026-04-26 user report):

1. Can't return from free-fly to player view.
   CameraController.ToggleFly only swaps Fly↔Orbit, so a user who
   flew out of player mode landed in orbit (Holtburg) on
   toggle-back instead of the chase camera. Added
   ToggleFlyOrChase() helper that prefers Fly→Chase /
   Chase→Fly when _playerMode is true and a chase camera is
   available; falls back to the original Fly↔Orbit toggle for
   offline / pre-login flows. Wired into all three free-fly
   entry points: keyboard shortcut (Ctrl+Shift+F), Camera menu
   item, and DebugPanel button.

2. Shift while moving STOPS instead of dropping to walk.
   Root cause: InputDispatcher.IsChordHeld required
   _keyboard.CurrentModifiers to match chord.Modifiers EXACTLY.
   So with W bound as (W, None), holding W and then pressing
   Shift made CurrentModifiers=Shift mismatch chord (None) →
   IsActionHeld(MovementForward) returned false → Forward flag
   dropped → player stopped. Fixed by relaxing IsChordHeld:
   when chord.Modifiers is None, Shift is allowed to coexist
   (it's the retail walk-modifier). Other modifiers
   (Ctrl, Alt, Win) still mismatch strictly so Ctrl+W stays a
   distinct chord from W.

   +2 tests pinning the new permissive-Shift / strict-Ctrl
   semantics.

3. Backwards too slow when running.
   forwardCmdSpeed for the WalkBackward branch was hardcoded
   to 1.0; localY was hardcoded to -(WalkAnimSpeed * 0.65).
   Neither honored input.Run. With Run=true (default),
   backward now scales by runRate (~2.4×) so X = "run
   backwards" matches the forward run pace × the 0.65
   backward animation cycle ratio.

4. Strafe too slow when running.
   localX for SideStepLeft / SideStepRight was hardcoded to
   ±SidestepAnimSpeed regardless of Run. Same fix: when Run
   is held, scale by runRate so strafe at default speed
   matches the run-forward pace.

Tests: 1220 → 1222 (the two new IsChordHeld tests).

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

src/AcDream.UI.Abstractions/Input/InputDispatcher.cs

@@ -142,8 +142,18 @@ public sealed class InputDispatcher
 
     /// <summary>True iff the given chord's primary key is currently down on
     /// the appropriate device AND the keyboard's current modifier mask
-    /// equals the chord's required modifier mask exactly. Modifiers must
-    /// match precisely — Ctrl+A held does NOT count Shift+Ctrl+A as held.</summary>
+    /// matches the chord's required modifier mask. Match semantics:
+    /// <list type="bullet">
+    /// <item>If <see cref="KeyChord.Modifiers"/> is <see cref="ModifierMask.None"/>,
+    /// any Shift state is allowed — Shift is the retail walk-modifier and
+    /// must coexist with movement chords (W held + Shift pressed = run
+    /// gracefully drops to walk, not "stop"). Other modifiers
+    /// (Ctrl, Alt, Win) still mismatch strictly so Ctrl+W stays a
+    /// distinct chord.</item>
+    /// <item>If <see cref="KeyChord.Modifiers"/> includes any non-Shift
+    /// modifier (or includes Shift explicitly), the match is exact —
+    /// Ctrl+A held does NOT count Shift+Ctrl+A as held.</item>
+    /// </list></summary>
     private bool IsChordHeld(KeyChord chord)
     {
         if (chord.Device == 0)
@@ -160,7 +170,14 @@ public sealed class InputDispatcher
             // Unknown device — never held.
             return false;
         }
-        return _keyboard.CurrentModifiers == chord.Modifiers;
+        var current = _keyboard.CurrentModifiers;
+        if (chord.Modifiers == ModifierMask.None)
+        {
+            // K-fix3 (2026-04-26): permissive Shift handling for bare-key
+            // chords. See the XML doc above for rationale.
+            current &= ~ModifierMask.Shift;
+        }
+        return current == chord.Modifiers;
     }
 
     /// <summary>Inverse of <see cref="MouseButtonToKey"/>: decode a chord