Implement live accessibility announcements

The board speaks when files change under the user, per DESIGN/10 § Live
board announcements. BoardDiff is the pure snapshot summarizer (identity
sets for cards/lanes added/edited/moved/deleted — ids, not tallies, so
pro-m1's semantic commit engine can build on it; edited = rendered
content only, moved beats edited, implied events don't steal the
subject). BoardAnnouncer is the decision seam: focusOutcome computes the
vanishing-focus sentence and the walk-up-then-sideways recovery (next
lane by order, else previous, board container only when none remain,
never the trash); speech(for:) is the one-sentence precedence ladder —
raised condition > bracket completion > cleared condition > vanished
focus > digest — foreign-only for the last two rungs, so app-mediated
echoes stay silent.

BoardStore.land assembles ReloadFacts and posts exactly one sentence per
reload through the injectable announce outlet (AccessibilityAnnouncer,
medium priority, never interrupting). Selection recovery layers on top
of ItemReferenceSet re-resolution — survivors veto, the emptied
selection lands on the vanished item's lane and re-arms ⌘N's active-lane
memory. performWholesale(announcing:) arms a completion phrase consumed
by the closing reload — nil on every base bracket today; pro-m1 fills
git phrasings. Locks raised outside the reload path (vanished root,
unwritable location) announce through the same ladder, and the banner
strip is a labeled "Board status" container whose row labels are the
announced sentences (AccessibilityPhrases.bannerLabel — one string for
eye and ear).

Announcements classify at reload granularity (WatchOrigin) as a
deliberate interim: DESIGN/02's EchoLedger (per-file classification, the
announcer's specified input, git-free) was scheduled with the
auto-committer that the edition split moved to pro-m1 — filed on the
Redesign board for a ruling. 1533 unit tests green, both schemes build.

Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY

KanbanTests/KeyboardGrammarTests.swift

@@ -388,8 +388,13 @@ struct SelectionHeadTests {
         store.handleWatcherEvent(.treeChanged(.foreign))
         await store.awaitQuiescence()
 
-        #expect(store.selection.isEmpty)
-        #expect(store.transient.selectionHead == nil)
+        // The crossing dropped the head, which is this test's rule. What stands afterwards is
+        // 10-accessibility.md's vanishing-focus recovery, layered on top: the selection had emptied
+        // and the cursor's card had gone, so focus lands on the card's lane and takes the head with
+        // it (`BoardAnnouncerStoreTests`, where the recovery itself is pinned). The card's own head
+        // reference is gone either way, which is what "a container crossing is a vanish" claims.
+        #expect(store.selection.ids == [lane1])
+        #expect(store.transient.selectionHead == lane1)
     }
 }