fix(r3): judge each rel once per request — a side or review item that vanishes mid-request never 500s

booth-dev's race note after the merge: the compare route resolved each side
in _compare_side and again in _compare_ring, then ring.index(a) raised if the
file vanished (or was relinked outside the booth) between the two; the review
did the same through cring.index(f). The compare ring is now built once and
the sides are judged by membership of it. The review re-judges its item and
scans forward for the next comparable one (usually one step, no longer a
resolve of the whole ring per render); an item no longer comparable renders
the review without a Compare control, and C does nothing.

The contract records the once-per-request rule and that the phone-width wrap
covers doc.html's bar too. r3.toml: 59 rows, four re-anchored.

docs/contracts/r3_compare.contract.md

@@ -62,6 +62,13 @@ assumptions:
   stays in the review ring). EVERY compare link is built from it: the strip,
   the steps, the review's Compare control and the `back=compare` landing. So
   no navigation offers a pair that 404s, and a step walks over such an item.
+- **Each rel is judged ONCE per request** (booth-dev, after the merge). The
+  route builds the compare ring once and judges both sides by membership of
+  it. Resolving a rel twice lets a file that vanishes between the two reach a
+  lookup that raises, a 500. The review re-judges its own item first and scans
+  forward for the next comparable one; when its item is no longer comparable,
+  the review renders WITHOUT a Compare control (and `C` does nothing), never a
+  500.
 - The response carries, per side: the rel, its quoted url, ordinal, kind,
   caption, blurred, flagged and thumb. It also carries the compare ring as a
   filmstrip in RING ORDER (the view route's `film`, one line in the route's docstring),
@@ -142,9 +149,10 @@ assumptions:
   gap, never a border that comes out of one side's width. Two stages of
   different sizes would draw the same picture at two scales in Fit.
 - **At phone width (600px and below) a top bar that cannot hold its controls
-  WRAPS** instead of scrolling the page sideways or crushing a control. This
-  applies to the review's bar too, which gains the Compare control (only its
-  glyph below 600px). The review's bar was already full: a fogged booth
+  WRAPS** instead of scrolling the page sideways or crushing a control. The
+  rule is on `.vbar`, so it applies to every bar of that class: compare's, the
+  review's (which gains the Compare control, only its glyph below 600px), and
+  doc.html's. The review's bar was already full: a fogged booth
   overflowed it by 3px at 390px before r3.
 - **Each stage is the r2c stage:** Fit fills (up or down, contain, never
   cropped), or 1:1 at natural pixels with every pixel reachable. Drag pans a