feat(r2): C3 server side — 204 on an explicit JSON Accept, and back=view

- wants_json: true only for an exact `application/json` entry with q > 0.
  Absent, empty, wildcard, application/*, near misses, q=0 and malformed
  headers all fall through to the 303.
- The four mark routes share one exit, _mark_done: 204 with no body for the
  in-place client, otherwise _mark_redirect unchanged.
- back=view lands on /b/<name>/view?f=<rel>#rail, only for a media item of
  this booth. It is built from the resolved rel and never echoed. Anything
  else takes the no-`back` landing.
- tests/golden/r2_mark_303.json: 108 responses recorded from the PRE-R2
  code (6 route cases x back absent|marks x 9 non-JSON Accepts), replayed
  byte for byte (INV-4). Two mutations (q>=0, substring match) turn it red.
- The contract now states the q=0 rule.

tests/test_flow.py

@@ -151,3 +151,76 @@ def test_a_look_that_cannot_be_recorded_still_serves_the_page(tmp_path):
         b.chmod(0o755)
     assert r.status_code == 200
     assert not (b / ".seen").exists()
+
+
+# ---- C3: in-place judgment --------------------------------------------------
+
+GOLDEN = pathlib.Path(__file__).parent / "golden" / "r2_mark_303.json"
+
+
+def _seed(root: pathlib.Path) -> TestClient:
+    """The golden's fixture, byte for byte (see golden_gen in the R2 notes)."""
+    from booth.marks import declare_pick, write_note
+    root.mkdir(parents=True, exist_ok=True)
+    b = _booth(root, "g", {"a.png": PNG, "b b.png": PNG, "c.md": PNG})
+    declare_pick(b, "q", {"prompt": "Which?", "options": ["x", "y"]}, target="a.png")
+    set_flag(b, "a.png", True)
+    write_note(b, "a.png", "seed")
+    return TestClient(create_app(root, ttl_hours=24, start_sweeper=False),
+                      follow_redirects=False)
+
+
+def test_every_pre_r2_request_shape_gets_a_byte_identical_303(tmp_path):
+    """INV-4. The golden was recorded from the PRE-R2 code: every mark route,
+    with `back` absent and `back=marks`, under nine Accept headers that must
+    NOT count as asking for JSON. Status, every header, and the body must match
+    exactly — the no-JS guarantee lives in these bytes."""
+    import json
+    cases = json.loads(GOLDEN.read_text())
+    assert len(cases) == 108
+    for i, case in enumerate(cases):
+        c = _seed(tmp_path / str(i))
+        headers = {} if case["accept"] is None else {"accept": case["accept"]}
+        r = c.post(case["path"], data=case["form"], headers=headers)
+        got = {"status": r.status_code,
+               "headers": sorted([k.lower(), v] for k, v in r.headers.items()),
+               "body": r.content.decode("latin-1")}
+        want = {k: case[k] for k in ("status", "headers", "body")}
+        assert got == want, (case["path"], case["form"], case["accept"])
+
+
+@pytest.mark.parametrize("path,form", [
+    ("/b/g/answer", {"ask": "q", "choice": "y"}),
+    ("/b/g/note", {"target": "a.png", "text": "in place"}),
+    ("/b/g/flag", {"target": "b b.png", "on": "1"}),
+    ("/b/g/unmark", {"mark": "note-1"}),
+])
+@pytest.mark.parametrize("accept", ["application/json", "text/html, application/json;q=0.5"])
+def test_an_explicit_json_accept_gets_204_and_the_write_still_lands(tmp_path, path, form, accept):
+    """The in-place path: same write as the form, no redirect, no body."""
+    from booth.marks import marks_for
+    c = _seed(tmp_path)
+    before = [(m.id, m.shape, m.answer, m.text) for m in marks_for(tmp_path / "g")]
+    r = c.post(path, data=form, headers={"accept": accept})
+    assert r.status_code == 204 and r.content == b""
+    assert "location" not in r.headers
+    after = [(m.id, m.shape, m.answer, m.text) for m in marks_for(tmp_path / "g")]
+    assert after != before, "the write must happen exactly as for the form"
+
+
+@pytest.mark.parametrize("f,landing", [
+    ("a.png", "/b/g/view?f=a.png#rail"),
+    ("b b.png", "/b/g/view?f=b%20b.png#rail"),
+    ("c.md", "/b/g/#item-b%20b.png"),          # a doc is not in the review ring
+    ("gone.png", "/b/g/#item-b%20b.png"),      # not an item
+    ("", "/b/g/#item-b%20b.png"),
+    ("../../etc/passwd", "/b/g/#item-b%20b.png"),
+])
+def test_back_view_lands_on_the_review_only_for_a_media_item(tmp_path, f, landing):
+    """The JS-off fix for the bounce: a flag set at full size lands back at
+    full size. Anything that is not a media item in this booth falls back to
+    the booth page exactly as a form with no `back` does."""
+    c = _seed(tmp_path)
+    r = c.post("/b/g/flag", data={"target": "b b.png", "on": "1", "back": "view", "f": f})
+    assert r.status_code == 303
+    assert r.headers["location"] == landing