a11y(Table): a sortable column header never exposes its sort state

[IgorShevchik]

Found while working #454. Sorting a B24Table column changes what a sighted user sees — the header's icon flips — and tells a screen reader nothing at all.

What is missing

aria-sort appears zero times in src/runtime/components/Table.vue. The <th> elements at :610 and :693 carry no sort state, so a screen reader announcing a sortable column says only its name, both before and after the user activates it. There is no way to hear whether a table is sorted, by which column, or in which direction.

WAI-ARIA defines aria-sort on the header cell for exactly this: ascending, descending, none, or other.

How it surfaced

While adding sort coverage for #454 I measured aria-sort in the snapshots to check the sorted branch had been reached. Zero — and the branch had been reached; the icon differs between all three states now. The metric was wrong because the attribute does not exist. Worth stating plainly: this was found by an assertion of mine being wrong, not by looking for it.

Not a porting loss

Upstream's Table.vue does not emit aria-sort either — also zero occurrences. So this is inherited, and fixing it is a deliberate divergence rather than a port. Recording that here because .sync/PORTING.md §7 asks for the distinction: a later port of upstream's table must not quietly take it back out.

The fix

On the <th> of a sortable column, derived from what the header already reads:

column.getIsSorted()      // 'asc' | 'desc' | false
  → aria-sort="ascending" | "descending" | "none"

none matters as much as the other two: the attribute must be present on every sortable header, or a screen reader cannot tell "sortable and unsorted" from "not sortable". Headers that are not sortable should carry no aria-sort at all.

Acceptance

• Every sortable header has aria-sort, non-sortable headers have none.
• The value tracks the column state through all three values.
• A spec case per state — and note that before test: fourteen specs assert a default because the fixture never reaches the branch #454 the sorted branches were unreachable in all 35 renderEach entries, so a case that does not set sorting proves nothing here.

Priority: P2 — a keyboard and screen-reader user cannot perceive a state change the interface makes.

Related: #476, #473 — the same theme of a control whose state is visible but not announced.