fix(inplace,embed): input set back mid-flight, report inputs, ambiguous anchors

Four items owed after S5b, reported by design-dev during the anti-slop run:

- carry() measured a sent-then-changed form against its OLD DEFAULTS. An
  answer set back mid-flight to the value the page first showed read as
  untouched, and the swap put the just-saved value over it. A form sent and
  then changed is now measured against its sent snapshot (sentSet.snapOf).
- The embed's clean-batch reload saw only our own forms. A report's own
  inputs lost whatever the operator had typed into them. Unsaved text in
  any control we don't own now holds the reload, and the page says so.
- Two r2b.toml rows ("D3 a stored theme...", "D3 forced light...") matched
  twice, so they proved only by where the first match fell. Both are
  re-anchored, and scripts/mutation_check.py now refuses any anchor that
  matches more than once. A new tool control covers that.
- The r2_flow contract's C3 steps 2 and 4 now say what S5b superseded. U3
  gains the report-input rule.

Mutation rows: u3_submit_all +1, r2_submit_all +1. Four rows were
re-anchored onto the moved lines.

tests/test_flow_browser.py

@@ -1819,3 +1819,33 @@ def test_a_batch_whose_refresh_fails_keeps_the_page(browser, live):
     assert _chosen(b) == {"a1": "yes", "a2": "no"}
     assert same and draft == "not sent", (same, draft)
     assert "reload" in said.lower(), said
+
+
+def test_a_sent_pick_set_back_to_its_first_value_mid_flight_is_kept(browser, live):
+    """design-dev's report, 2026-09-28: carry() measured a changed-after-press
+    form against its OLD DEFAULTS, so an answer set back to the value the page
+    first showed looked untouched — and the swap put the just-saved value
+    back over the operator's last word, silently. A sent form that changed
+    since its press is now measured against what it SENT."""
+    from booth.marks import answer_pick
+    base, root = live
+    b = _set(root, 1)
+    _picks(b, ("a1",))
+    answer_pick(b, "a1", "no")
+    page = browser.new_page()
+    page.add_init_script(_HOLD_POSTS)
+    page.goto(f"{base}/b/g/marks", wait_until="networkidle")
+    page.locator("#mark-a1 summary.mark-change").click()
+    page.check('#mark-a1 input[type=radio][value="yes"]')
+    page.locator("#mark-a1 .mark-submit").click()
+    page.wait_for_timeout(150)
+    page.check('#mark-a1 input[type=radio][value="no"]')      # back, mid-flight
+    page.wait_for_function(
+        "document.querySelector('#mark-a1 .mark-answer-choice') && "
+        "document.querySelector('#mark-a1 .mark-answer-choice').textContent.trim() === 'yes'",
+        timeout=10000)
+    page.wait_for_timeout(300)
+    showing = page.is_checked('#mark-a1 input[type=radio][value="no"]')
+    page.close()
+    assert _chosen(b) == {"a1": "yes"}
+    assert showing, "the value set back mid-flight was replaced by the saved one"