v0.4.3 — finish the textarea

v0.4.3 — finish the textarea

Closes the gaps v0.4.2 shipped with. No new widgets, and the routed-message
contract is unchanged.

Added:

• Goal-column memory for textarea vertical motion. Moving down through a
  short line and back returns to the column you started in; previously each
  motion re-derived the column, so one short row truncated it permanently.
  The column is runtime state, keyed by focus — it never enters the public
  message contract. Harlock.TextArea.apply_key/6 threads it.
• Harlock.TextArea.expand_tabs/2. A tab is zero-width to Harlock.Width,
  so a \t in a value holds a cursor index while consuming no display cell.
  Tabs can only arrive from bracketed paste or an app assigning :value,
  both app-side, so this is a helper rather than enforcement at insert.
• Alt-Backspace kills the word backward in text_input and textarea.
• examples/notes.exs — the first runnable demo of the widget.

Changed:

• Widget key routing carries runtime state back to the caller, so a key that
  changes interaction state without changing value or cursor records it and
  still falls through to update/2. Internal; no app-visible change beyond
  goal columns.

Fixed:

• ROADMAP no longer lists a missing row.ex helper. Element.Column is a
  table-column spec, not a layout container.

Full notes in CHANGELOG.md.