fix(asks): one submit saves every ask on the page

Operator report (via infra-ops): on a page with several asks, a submit
saved only the pressed one and the reload wiped the rest. Confirmed on
auk-audition: one POST at 15:02:23 saved the last ask on the page, then a
400 from the submit of an ask the reload had just blanked.

Client-side on both surfaces; /answer is unchanged. A submit on a pick
form, while another pick form on the page holds unsent input, sends every
changed ("dirty") pick form: one POST each, to its own action, with
Accept: application/json, in document order. A refusal stops nothing, and
untouched forms are never re-sent. With no other dirty form, a submit is
exactly what it was.

- embed.js (verbatim reports): reloads only when nothing was refused and
  nothing of ours is dirty. Otherwise a server-rendered status line in the
  submit block says what did not save, and input stays. A form the server
  took gets a new baseline. A press during the flight is ignored.
- base.html (marks page, lightbox, review rail): one refresh in place. A
  batch never reloads. Only forms the server took count as sent. In-flight
  state and "just sent" are keyed by form identity (formKey) plus the fields
  at the press, not the DOM node.

Two heid bug-hunt rounds: a four-arm panel on the first cut, then Hulda
alone on the fold. Ten findings reproduced red in a browser before their
fixes. Contracts: U3 "Submitting several asks at once" + INV-8, R2 C3
steps 2, 3 and 3a. Mutation tables u3_submit_all (15) and r2_submit_all
(11), all proved. Suite 928 -> 951.

tests/mutations/r2_flow.toml

@@ -118,7 +118,7 @@ new = '''        except OSError:
 label = 'C3 client: no busy guard (a double-click writes twice)'
 file = "booth/templates/base.html"
 test = "tests/test_flow_browser.py::test_an_unsaved_choice_survives_a_save_elsewhere_and_a_double_click_writes_once"
-old = '''      if (form.__busy) return;
+old = '''      if (pending[flightKey(form)]) return;
 '''
 new = ''''''
 
@@ -133,8 +133,8 @@ new = ''''''
 label = 'C3 client: saves are not serialized'
 file = "booth/templates/base.html"
 test = "tests/test_flow_browser.py::test_quick_successive_flags_all_show"
-old = '''      queue = queue.then(function () { return run(form, data); })'''
-new = '''      queue = run(form, data)'''
+old = '''      queue = queue.then(function () { return run(form, data, snap); })'''
+new = '''      queue = run(form, data, snap)'''
 
 [[mutation]]
 label = 'C3 the standalone marks page has no region'