v0.5.0 — Reactive Bindings

Two-way binding between rendered preview and the .grid data section.

Inline cell binding via Jinja helper:

<td>{{ cell("B2") }}</td>            <!-- editable -->
<td>{{ cell("B2@sales") }}</td>      <!-- cross-sheet -->

Form-style bind: blocks:

bind: input
  cell: B2
  type: number

bind: select
  cell: A2
  options: North, South, East, West

• POST /api/cell-edit accepts {cell: "B2", value, sheet, save}
• apply_edit() rewrites only the target row — preserves comments, blank lines, @Directives, formulas in other cells
• Header row (row 1) always read-only
• 73 new tests including HTTP roundtrip via stdlib http client

See spec/SPEC.md §18.