feat(replay): wire ← / → keyboard accelerators for paused stepping

→ during a paused replay advances by one move (mirrors the Stop
button's existing forward-step semantics). ← decrements the
cursor and dispatches `UndoRequestEvent`, which the game's
`handle_undo` reads next frame to reverse its most-recent move
— hooking the existing undo system rather than replaying
forward from cursor 0 (every replay-applied move pushes to the
undo stack the same way a player move would, so undo is the
right reversal primitive).

Both accelerators are paused-only — backwards via a new
`step_backwards_replay_playback` in `replay_playback.rs` that
hard-gates with the same destructure pattern as
`step_replay_playback`. Pressing → during running playback or ←
at cursor 0 are silent no-ops; the player learns "pause first,
then arrow."

The mockup labels these `[← →] scrub` (continuous fast scan).
Single-move step is the closest behaviour shippable today —
continuous scrub would need either a key-held event source or
an internal speed-up loop. Footer hint reads
`[← →] step` to match what's wired rather than the aspirational
"scrub."

Footer hint extended in lockstep:
`[SPACE] pause/resume · [ESC] stop · [← →] step` — the
only-wired-keybinds discipline holds.

ReplayOverlayPlugin gains `add_message::<UndoRequestEvent>()`
defensively so the plugin can run under MinimalPlugins without
GamePlugin attached (idempotent registration; harmless when
GamePlugin is also present).

6 new tests (2 hint pins + 4 keyboard scenarios) + 1 helper-pin
update for the new hint string.

Pre-existing flake noted: `daily_challenge_plugin::tests::
check_system_fires_warning_event_only_once_per_day` is failing
because wall-clock UTC is currently within 30 minutes of
midnight, inside the daily-expiry warning window the test
asserts against. Verified pre-existing by stashing all changes
and re-running — failure persists. Same shape as the
`winnable_seed_search` flake the handoff documented earlier
this session: time-dependent, deterministically passes under
different clock conditions. Not introduced by this commit.

Clippy clean.

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

solitaire_engine/src/replay_playback.rs

@@ -42,7 +42,7 @@
 use bevy::prelude::*;
 use solitaire_data::{Replay, ReplayMove};
 
-use crate::events::{DrawRequestEvent, MoveRequestEvent, StateChangedEvent};
+use crate::events::{DrawRequestEvent, MoveRequestEvent, StateChangedEvent, UndoRequestEvent};
 use crate::game_plugin::{GameMutation, RecordingReplay};
 use crate::resources::GameStateResource;
 use crate::settings_plugin::SettingsResource;
@@ -284,6 +284,52 @@ pub fn step_replay_playback(
     true
 }
 
+/// Steps the replay **backwards** by exactly one move while paused.
+///
+/// Strategy: the live game's undo system is the source of truth for
+/// reversing moves. Every move the replay forward-stepped (via
+/// [`step_replay_playback`] or the auto-advance loop in
+/// [`tick_replay_playback`]) was dispatched as a canonical
+/// [`MoveRequestEvent`] / [`DrawRequestEvent`], which the game
+/// applied and pushed onto its undo stack. So a backwards step here
+/// is simply: decrement the cursor (so the about-to-apply move
+/// re-points at the one we're rewinding past) and fire an
+/// [`UndoRequestEvent`] so the game reverses its most-recent move
+/// next frame.
+///
+/// Hard-gated to the paused state via destructure pattern —
+/// matches the existing [`step_replay_playback`] gate so the
+/// player can only scrub one direction at a time and the tick
+/// loop never races a manual rewind.
+///
+/// Returns `false` and is a no-op in three cases:
+/// - State isn't `Playing` (no replay attached).
+/// - State is `Playing` but not paused (the tick loop owns the cursor).
+/// - Cursor is already at 0 (nothing to rewind past).
+///
+/// Returns `true` on a successful step; the actual game-state
+/// reversal happens next frame when `handle_undo` reads the
+/// `UndoRequestEvent`.
+pub fn step_backwards_replay_playback(
+    state: &mut ResMut<ReplayPlaybackState>,
+    undo_writer: &mut MessageWriter<UndoRequestEvent>,
+) -> bool {
+    let ReplayPlaybackState::Playing {
+        cursor,
+        paused: true,
+        ..
+    } = state.as_mut()
+    else {
+        return false;
+    };
+    if *cursor == 0 {
+        return false;
+    }
+    *cursor -= 1;
+    undo_writer.write(UndoRequestEvent);
+    true
+}
+
 /// Tick system. Runs every frame; only does work when
 /// [`ReplayPlaybackState::is_playing`].
 ///