docs(handoff): record win_move_index data field; B's next finite step

Captures `ab857bb` (Replay::win_move_index data field) under "Since
the v0.21.3 cut". Updates the Visual-identity follow-up entry for
B-2 to flag the data-layer prerequisite as landed and identifies
the WIN MOVE scrub-bar marker UI as the natural next finite commit.
Bumps test count to 1212.

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

SESSION_HANDOFF.md

@@ -21,15 +21,16 @@ resume.
 ## Status at pause
 
 - **HEAD locally:** see `git rev-parse HEAD`. The cut commit is
-  `3d92a91`; any post-cut docs edits ride on top of that.
+  `3d92a91`; the post-cut `Replay::win_move_index` data field
+  (`ab857bb`) rides on top of that.
 - **HEAD on origin:** matches local. v0.21.3 is fully on origin.
 - **Working tree:** clean. No WIP outstanding.
 - **`artwork/` directory:** still untracked. Intentional.
 - **Build:** `cargo clippy --workspace --all-targets -- -D warnings`
   clean.
-- **Tests:** **1207 passing / 0 failing** across the workspace
-  (net +12 from v0.21.2's 1195 baseline). Detail in
-  `CHANGELOG.md` § [0.21.3] § Stats.
+- **Tests:** **1212 passing / 0 failing** across the workspace
+  (1207 from v0.21.3's stats + 5 from `ab857bb`'s
+  `win_move_index` coverage).
 - **Tags on origin:** `v0.9.0` through `v0.21.3`. v0.21.3 is on
   `3d92a91`; v0.21.2 stays on `f23df3b`; v0.21.1 stays on
   `daa655a`; v0.21.0 stays on `04f9bf9`; v0.20.0 stays on
@@ -37,9 +38,21 @@ resume.
 
 ## Since the v0.21.3 cut
 
-No threads in flight. Working tree clean as of 2026-05-08. New
-work since the cut would land here as commit narratives; for
-the v0.21.3 contents themselves, see `CHANGELOG.md` § [0.21.3].
+- **`ab857bb` — `Replay::win_move_index` data field landed.**
+  First finite step toward the B-2 replay screen-takeover
+  redesign. Additive optional `Option<usize>` on `Replay` with
+  `#[serde(default)]` so older `latest_replay.json` /
+  `replays.json` files load unchanged (no schema bump). Populated
+  at the live recording site via a new `with_win_move_index`
+  builder; for fresh recordings the value is always
+  `Some(moves.len() - 1)` because recording freezes on win, but
+  storing it explicitly lets the playback UI read the WIN MOVE
+  position directly without re-deriving on every render. UI
+  consumption (the WIN MOVE marker on the scrub bar, plus the
+  broader screen-takeover work — move-log scroller, mini-tableau
+  preview, playback controls) lands in subsequent commits. 5 new
+  tests (1207 → 1212): default, builder set / set-None, on-disk
+  round-trip, legacy-JSON-loads-with-None backward-compat.
 
 ## Open punch list
 
@@ -77,11 +90,14 @@ palette refresh all shipped in v0.20.0 + v0.21.0. What stays open:
   a WIN MOVE marker on the scrub bar. Banner-local pieces all
   shipped in v0.21.0 (`c84d9f4` + `6204db8` + `54005d5` +
   `e080b49`); the floating MOVE chip above the focused card
-  shipped in v0.21.2 (`2fb2d63`). The screen-takeover is a
-  multi-session redesign with data-layer impact — needs a new
-  `win_move_index: Option<usize>` field on `Replay` (currently
-  unimplemented), a move-log scroller, and a mini-tableau
-  preview.
+  shipped in v0.21.2 (`2fb2d63`). The data-layer prerequisite
+  for the WIN MOVE marker — `Replay::win_move_index:
+  Option<usize>` — landed post-v0.21.3 in `ab857bb` (additive
+  serde-default field; populated at the recording site;
+  exhaustively tested). What still needs to land for the
+  screen-takeover: the WIN MOVE scrub-bar marker UI (consumes
+  the new field), a move-log scroller, a mini-tableau preview,
+  and playback controls. Multi-session.
 - *Floating `MOVE N/M` chip above the focused card during
   playback — closed 2026-05-08 by `2fb2d63`.* World-space
   `Text2d` entity sibling to the banner overlay; uses the same
@@ -258,10 +274,15 @@ DECISION TO ASK THE PLAYER FIRST:
      and Android Keystore stubs that need real bridges. Larger
      scope; needs an Android device or emulator running.
   B. Replay-overlay screen-takeover redesign — multi-session
-     work: move-log scroller, mini-tableau preview, WIN MOVE
-     marker on the scrub bar (needs new `Replay::win_move_index`
-     field), playback controls. The smaller floating-MOVE-chip
-     piece of B already shipped in v0.21.2 (`2fb2d63`).
+     work: WIN MOVE marker on the scrub bar (data field
+     `Replay::win_move_index` already landed in `ab857bb`, just
+     needs UI consumption), move-log scroller, mini-tableau
+     preview, playback controls. The smaller floating-MOVE-chip
+     piece of B already shipped in v0.21.2 (`2fb2d63`); the
+     data-layer prerequisite shipped post-v0.21.3 in `ab857bb`.
+     The natural next commit on B is the WIN MOVE marker UI on
+     the scrub bar — finite, single-system, consumes the new
+     field directly.
   C. Phase 8 (sync) — local storage scaffolding, self-hosted
      Axum server, `SolitaireServerClient` impl, GPGS stub
      wired into Settings. The biggest open arc by scope; rolls