fix+refactor+docs: P0–P3 todo list items

P0 fixes:
- Register WinSummaryPlugin, SelectionPlugin, CardAnimationPlugin in main.rs
  (all three were exported but never wired — features silently did nothing)
- game_state::draw(): increment move_count on waste→stock recycle, not just
  on normal draws; add move_count_increments_on_recycle regression test

P1 fixes:
- solitaire_server/Cargo.toml: remove duplicate dev-dependencies
  (solitaire_sync, uuid, chrono, jsonwebtoken were in both sections)

P2 — input_plugin refactor:
- Split 198-line handle_keyboard() into three focused systems under 110 lines each:
  handle_keyboard_core (U/N/Z/D/Space), handle_keyboard_hint (H), handle_keyboard_forfeit (G)
- Introduce KeyboardConfirmState resource to share countdown timers across systems
- Add three new unit tests: all_hints_suggests_draw_*, all_hints_is_empty_when_truly_stuck,
  new_game_confirm_window_is_positive

P2 — achievement predicate tests (solitaire_core):
- Add 10 direct unit tests for speed_demon, lightning, no_undo, high_scorer,
  on_a_roll, comeback predicates (previously only covered via check_achievements())
- 141 core tests now passing

P2 — server tests:
- solitaire_server/src/sync.rs: 4 unit tests for merge logic (no DB required)
- solitaire_server/src/leaderboard.rs: 2 unit tests for entry shape and sort order

P3 — documentation:
- Add struct-level ///  to 12 Plugin structs (ChallengePlugin, CursorPlugin,
  AnimationPlugin, HelpPlugin, PausePlugin, AudioPlugin, DailyChallengePlugin,
  HudPlugin, LeaderboardPlugin, OnboardingPlugin, TimeAttackPlugin, WeeklyGoalsPlugin)
- Add field-level /// to Card, Pile, Deck, GameState, AchievementContext, AchievementDef
- Add /// to WeeklyGoalKind, WeeklyGoalDef, WeeklyGoalContext, StatsExt::update_on_win

card_animation module (new files from previous session):
- chain.rs, diagnostics.rs, tuning.rs, updated interaction.rs/animation.rs/mod.rs/lib.rs
- Remove unused HOVER_SCALE_DEFAULT / DRAG_LIFT_SCALE_DEFAULT / HOVER_LERP_SPEED_DEFAULT constants
- Add handle_touch_stock_tap so touch users can draw from the stock pile

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/src/card_animation/animation.rs

@@ -140,10 +140,22 @@ impl CardAnimation {
 
 /// Redirects a card to a new destination without snapping or interrupting motion.
 ///
-/// Reads the card's current interpolated position (from a live `CardAnimation` if
-/// present, or from `Transform` if the card is stationary) and starts a fresh
-/// `CardAnimation` from that position. Duration is recalculated from the remaining
-/// distance so short remaining paths feel appropriately quick.
+/// Reads the card's current interpolated position (from a live [`CardAnimation`]
+/// if present, or from `Transform` if stationary) and starts a fresh
+/// [`CardAnimation`] from that position. Duration is recalculated from the
+/// remaining distance so short paths stay quick.
+///
+/// # Velocity continuity
+///
+/// When a card is mid-flight, the new animation starts with a small positive
+/// `elapsed` offset (`carry`) derived from how far through the current animation
+/// the card is. This preserves a sense of forward momentum: the new curve does
+/// not restart from zero velocity, avoiding a visible "lurch" when the target
+/// changes rapidly.
+///
+/// The carry is deliberately small (≤ 10 % of the new duration) so that it
+/// never causes a visible position jump — the card's start position is still
+/// read from the current transform.
 ///
 /// # Example
 ///
@@ -169,17 +181,29 @@ pub fn retarget_animation(
     new_end_z: f32,
     curve: MotionCurve,
 ) {
-    let (current_xy, current_z) = match current_anim {
-        Some(anim) => (anim.current_xy(), transform.translation.z),
-        None => (transform.translation.truncate(), transform.translation.z),
+    let (current_xy, current_z, momentum_carry) = match current_anim {
+        Some(anim) if anim.duration > 0.0 => {
+            // Estimate how far into the current animation we are and carry
+            // a small fraction of that progress into the new animation.
+            // This avoids restarting from zero velocity and makes the motion
+            // feel continuous when the target changes mid-flight.
+            let t = (anim.elapsed / anim.duration).clamp(0.0, 1.0);
+            // Cap at 10 % of the new animation so there's no visible jump.
+            let carry = (t * 0.12).min(0.10);
+            (anim.current_xy(), transform.translation.z, carry)
+        }
+        _ => (transform.translation.truncate(), transform.translation.z, 0.0),
     };
 
     let distance = current_xy.distance(new_end);
+    let duration = compute_duration(distance);
+
     commands.entity(entity).insert(CardAnimation {
         start: current_xy,
         end: new_end,
-        elapsed: 0.0,
-        duration: compute_duration(distance),
+        // Start slightly into the new animation to carry forward momentum.
+        elapsed: momentum_carry * duration,
+        duration,
         curve,
         delay: 0.0,
         start_z: current_z,