BROWSE lookup dropdown + membership enforcement in grid-edit and REPLACE #60

Part of the v1.3.0 lookup feature. Design: docs/superpowers/specs/2026-07-10-lookup-columns-design.md (on release/v1.3.0). Builds on the LOOKUP qualifier issue.

Scope

• grid-open columnTypes entries gain lookup + resolved options
• Grid.ts: lookup columns edit via <select>; static cells keep showing the stored value (labels only inside the dropdown — the grid must not disagree with LIST/report output)
• cellValidation.ts: membership check (exact, case-sensitive), running client-side and in Session's authoritative grid-edit re-check — the established two-sided pattern
• REPLACE enforces membership on columns that declare a lookup (additive: no existing column declares one, so no existing program changes behavior)
• Server re-resolves table lookups at write time (client lists may be stale)
• Unresolvable/over-ceiling lookup: free text + warn, membership not enforced; APPEND RECORD still leaves NULL

Tests

• Vitest: membership accept/reject via grid-edit and REPLACE; degradation cases (dropped source table must not lock the column)
• Playwright: grid dropdown asserted with toBeInViewport() (the demos/overtime.prg — Overtime Tracking demo #46 clipping trap — a <select> inside an overflow: hidden cell); a rejected off-list REPLACE; screenshot inspected before believing it works