TUI: make order book depth levels interactive (toggle preset + scroll through levels)

[llcro]

Summary

Add keybindings in the Market detail view so the user can (1) toggle the fetched order-book depth between presets, and (2) walk the selection up and down through rendered levels to inspect cumulative liquidity at specific price points. Today the book is fixed at depth=20 with no interactive way to see deeper levels or compare liquidity across levels.

Current behaviour

• crates/cdcx-tui/src/tabs/market.rs:160 hardcodes {"depth": "20"} on the REST public/get-book fetch.
• crates/cdcx-tui/src/widgets/detail_view.rs:97 (draw_book) renders all bids/asks it has, with no selection cursor.
• User can see only top 20 bids + 20 asks. No way to:
  • Zoom out to see top 50 / 150 for whale/sweep analysis.
  • Zoom in to top 5 for tight-spread instruments.
  • Scroll through levels to read cumulative depth at a specific price.

Proposed feature (two complementary pieces)

Piece 1 — Depth preset toggle

Keybinding (suggested: D or [/] pair) cycles the fetched depth through presets: 5 / 10 / 20 / 50 / 150.

• Preset is persisted in AppState so it survives instrument switches during the session.
• On change: re-fetch via public/get-book with new depth, re-subscribe to book.{instrument}.{depth} if the WS channel supports depth suffixes (verify — some exchanges require book.symbol.N while others push a fixed depth).
• Footer hotkey hint line gets a D:depth(50) indicator showing the current preset.

Piece 2 — Level navigation

Selection cursor that the user moves with ↑/↓ or j/k:

• Cursor lands on a specific bid or ask row.
• Highlighted row shows: price, level qty, cumulative qty from top of book to this level, cumulative notional USD, and distance from mid in bps/%.
• Pressing Esc / switching view clears the selection.
• Optional: pressing Enter on a selected level pre-fills the place-order modal with that price.

Piece 2 on its own is independently useful even without Piece 1 — scrolling through 20 levels helps analyse thin books. But they pair naturally: toggle to 150, scroll to find the liquidity wall.

Implementation notes

• Piece 1 is mostly plumbing: add book_depth: usize to MarketTab (or AppState if we want it persisted via the tui.toml workflow from TUI: watchlist add/remove does not persist to ~/.config/cdcx/tui.toml #23), change the fetch params, add a key handler, update the status line.
• Piece 2 needs:
  • A book_cursor: Option<usize> on MarketTab, plus a convention for index-into-concatenated-asks-then-bids.
  • A detail pane under/beside the book showing the cumulative stats for the selected level.
  • Care in draw_book to style the selected row without breaking the existing depth-bar rendering (detail_view.rs:194-251).

Both pieces are agent-parity opportunities — a matching MCP tool should be able to query the book at a given depth (already possible via cdcx market book) so agents can reach the same insight the TUI now surfaces.

Scope

One issue covering both pieces; the PR can land them separately. Piece 1 first is the lowest-friction starting point.

Related

• Surfaced during audit for TUI: Recent Trades panel replaces instead of accumulating #21 (Recent Trades).
• TUI: watchlist add/remove does not persist to ~/.config/cdcx/tui.toml #23 (watchlist persistence) would provide the tui.toml round-trip needed if we want the depth preference to persist across sessions.