feat(tui): interactive book level cursor with cumulative readout

[llcro]

Closes #26 (Piece 2 — level-scroll cursor + cumulative-depth readout). Piece 1 shipped in #31.

Summary

• ↑/↓ on Market Detail activates a cursor and walks through levels. Crosses the mid between best ask and best bid, clamps at the deepest visible level on each side (no wrap).
• Selected row is highlighted (bg + bold).
• Bottom line of the book panel swaps from the pressure bar to a cumulative readout when cursor is active: `ASK fix(tui): fill candle gaps for illiquid instruments #3 p=78,550.2 q=0.123 cum_q=0.543 cum_$=42,683 Δmid=+12.4bps`.
• Cumulative notional = Σ (price × qty) per level, not `cum_qty × last_price` — the level-by-level sum matches what the user expects when prices vary across levels.
• Double-Esc pattern: first press clears cursor (stays on Detail); second press exits Detail. Gives users a non-destructive way to dismiss the readout.
• Cursor also clears on `D` (depth change invalidates index), `k`/`m` (view switch), instrument switch, or when the side empties out via WS update.
• Help overlay "Detail View" page documents ↑↓ + new Esc semantics.

Test plan

• `cargo test -p cdcx-tui --lib` → 73 passed, 11 new, 0 failed
• `cargo fmt --all --check` clean
• `cargo clippy --workspace --all-targets` clean
• Mutation-verified: breaking `Ask(0) → Bid(0)` crossing fails `cursor_down_from_best_ask_crosses_to_best_bid`; replacing per-level notional with `cum_qty × last_price` fails `cumulative_at_sums_notional_level_by_level`
• Manual QA: activate cursor with ↓, walk through both sides, verify clamping at deepest bid/ask, verify double-Esc, verify `D` clears cursor, verify cumulative numbers match the book's Total column row-for-row

Design decisions worth flagging

• Pure helpers: `cursor_up`, `cursor_down`, `clamp_cursor`, `cumulative_at`, `bps_from_mid` are standalone functions testable without `AppState`. The cursor movement contract is encoded in tests, not implicit in the render path.
• Title indicator: panel title becomes `"Order Book (50) — cursor"` when active, so the pressure-bar swap isn't surprising.
• Scoped out of this PR: Enter-on-cursor to prefill the place-order modal (natural follow-up, file as separate issue). Mouse click to set cursor (UX polish). MCP tool exposing the same cumulative stats (agent-parity follow-up — reuses the pub helpers already shipped here).