copy: the thinking-field placeholders, the brief round (release polish 5)

Four placeholders rewritten per the owner's set (2026-10-07): the
thinking fields are an invitation to stream-of-consciousness transfer,
and everything else is implied by the field's existence — no "the
teacher reads all of this", no mechanism, no em dashes, nothing that
wraps. The reading field's placeholder no longer breaks to a second
line.

  quiz:     "Why this answer? Whatever comes to mind."
  reading:  "Whatever comes to mind as you read."
  practice: "Work here, think out loud."
  pause:    "Ask anything, or press Enter for the next one."

The answer-field placeholders ("Type your answer…") stay: brief and
functional already (owner ruling). Quiz placeholder pin updated.

1428 tests, tsc, lint, build green.

tests/render-cards.test.tsx

@@ -143,7 +143,7 @@ describe("render smoke tests", () => {
 		// INSIDE the dock — never called optional, inviting the dump.
 		assert.ok(html.includes('data-testid="thinking-field"'), "thinking field present");
 		assert.match(html, /YOUR THINKING/, "labeled as the learner's voice (all caps, owner round 3)");
-		assert.match(html, /Why this answer\? What are you unsure about\?/, "placeholder invites, not apologizes");
+		assert.match(html, /Why this answer\? Whatever comes to mind/, "placeholder invites, not apologizes (owner, placeholder round 2026-10-07)");
 		assert.ok(!html.includes("optional"), "the field is never called optional");
 		assert.match(html, /Submit answer/, "send becomes Submit answer");
 		assert.match(html, /↵ submits · ⇧↵ newline/, "the standard chord is discoverable (owner round 5)");