fix(review): the heid code-review and bug-hunt panels on r2c, folded (both 4/4 with retries)

- 1:1 start-aligns. The centred flex item overflowed both sides and the
  start was unreachable; measured, a 3000px picture hid its leftmost
  980px. Auto margins still centre a small picture.
- Drag lifecycle: a move with no button ends the drag, so a press
  released outside the stage never pans on a later hover. Capture is now
  load-bearing in a test. The threshold is 4px of total movement.
- A press on the stage's own scrollbar is never a pan. The arrows clamp
  to the stage's client box, so they are never under a classic
  scrollbar. The test runs a browser without --hide-scrollbars and
  asserts the gutter exists.
- Stacked, the arrows' CSS spot is the stage's centre (30vh), set in
  view.html because base.html lost to the page's later rule.
- The stage reveal is `hidden` until bound, and keeps Fit's drop shadow
  when revealed. A blurred picture composes blur() drop-shadow().
- The mode follows another tab. A failed or unknown size returns the
  arrows to their CSS spot.
- Tests: object-position, vertical centring, the Fit half of
  aria-pressed, a storage read that throws, a large picture's toggle,
  Fit forgetting 1:1, single-axis pan.
- Declared: the r2b reveal test reads "no blur" (the shadow stays), and
  the r2_flow 360px-offset row is retired.

Mutation tables 137/137 across four. 810 passed.

tests/mutations/r2_flow.toml

@@ -11,6 +11,10 @@
 # covers them together); and `flagged_targets`' `error is None`, since
 # hydration already strips the target from a damaged mark.
 #
+# RETIRED (r2c S3, 2026-09-23): the row on the next arrow's 360px rail offset —
+# the arrows now sit at the drawn picture, clamped inside the stage; the test was
+# replaced as declared in r2c's contract, and its successors are in r2c.toml.
+#
 # RETIRED (r2b D1, 2026-09-23): four rows proving the facts-line row controls
 # (visible at rest, compact, on the facts line) — the operator ruled those
 # controls hover-revealed over the preview strip, and their tests were replaced
@@ -139,16 +143,6 @@ test = "tests/test_flow_browser.py::test_the_standalone_marks_page_updates_in_pl
 old = '''<div class="marks-panel" data-region="marks-panel">'''
 new = '''<div class="marks-panel">'''
 
-[[mutation]]
-label = "C6 the next arrow's rail offset applies at phone width"
-file = "booth/templates/view.html"
-test = "tests/test_flow_browser.py::test_the_next_arrow_clears_the_rail_only_beside_it"
-old = '''  .vprev{left:0}.vnext{right:0}
-  @media (min-width:901px){.vnext{right:360px}}
-'''
-new = '''  .vprev{left:0}.vnext{right:360px}
-'''
-
 [[mutation]]
 label = "resolver: an entry that cannot be stat'd raises out of booth_items"
 file = "booth/items.py"