Footprint chart + delta/CVD #38

Part of the percept.one parity roadmap.

Depends on: trade data model issue (+ custom series API from the extension points issue).

Spec

Bid×ask traded volume per price cell inside each bar, per-bar delta, and cumulative volume delta (CVD) as a sub-pane indicator.

Work

• Footprint as a custom series type (validates the new registerSeries API)
• Cell text rendering with readable density rules (collapse to profile bars when zoomed out)
• CVD + delta sub-pane indicators via the existing indicator registry

Completion audit

Verified on main in merged PR #41: createFootprintSeries registered as a custom series, bid×ask text with density-based profile/delta fallback, and delta/CVD sub-pane indicators through the existing registry. Covered by packages/chart/test/footprint.test.ts.

Validation: the existing main suite passed 1,849 tests with 100% line, branch and function coverage; PR #41 main coverage and documentation workflows both passed.