fix(r3): fold heid's code review — equal stage widths, the axis guard, players, and tests that read the observable

The one drift: the separator was a border on B, making B's stage 1px
narrower than A's; it is now a 1px column gap, so the stages are the same
size to the pixel. Tests now read what the contract promises instead of a
proxy: the strip's ring order, the full bakeoff sequence, 1:1 and Fit by
geometry, the 900px break from both sides, A wrapping, each form naming its
own item, a sibling-prefix symlink, both reveals, the strip's flag, the back
arrow unlinked, a one-axis picture, a focused player, two videos with no
toggle. The contract names .cmp-cap, a press on a stage, INV-4's URL-driven
picker and the redirect branch's isinstance check. r3.toml gains ten rows.

docs/contracts/r3_compare.contract.md

@@ -47,7 +47,8 @@ assumptions:
 - **A missing or empty param is a 404, not FastAPI's 422.** The review
   declares `f: str` and so answers 422 when `f` is absent. Compare declares
   `a: str = ""` and `b: str = ""`, 404s an empty one, and checks each with the
-  same `isinstance(str)` the view branch gives `f`.
+  same `isinstance(str)` that `_mark_redirect`'s `back=view` branch gives `f`
+  (the view route itself declares `f: str` and checks nothing more).
 - **Route order:** the compare route is registered BEFORE the catch-all
   `/b/{name}/{filepath:path}`, as view is. Accepted, and written down: a booth
   FILE literally named `compare` is unreachable at `/b/<name>/compare`. This
@@ -71,8 +72,9 @@ assumptions:
   - Each frame is marked `A`, `B`, or nothing (both marks when `a == b`).
   - With JS, a click on a frame sets the ACTIVE side to that item and stays on
     the compare page. The active side defaults to B.
-  - The active side wears the SVOS reticle (the one selection device). A click
-    on either stage, or the key `X`, makes that side (or the other) active.
+  - The active side wears the SVOS reticle (the one selection device). A press
+    on either stage (pointerdown, so starting a 1:1 pan there also makes it
+    active), or the key `X`, makes that side (or the other) active.
   - **The active side lives on a NON-region element**, the side's wrapper
     around its stage, and in the URL's `side` (C2), rewritten in place when it
     changes. An in-place save swaps regions (strip, labels, flags), and
@@ -131,8 +133,9 @@ assumptions:
   review's.
 - **The two stages are always the same size.** The sides share one set of rows
   (subgrid), so a caption under one side takes its height from both stages,
-  never from that side's alone. Two stages of different sizes would draw the
-  same picture at two scales in Fit.
+  never from that side's alone, and the separator between them is a column
+  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
@@ -245,8 +248,9 @@ assumptions:
   - compare.html carries `{% block html_attrs %} data-booth="{{ name }}"`,
     because without it Reveal all's script and the head script's reveal
     restore both bail (S6, r2b's mutation row for the review).
-- **Each side's caption** shows under its stage, clamped to 20vh with its own
-  scroll, the same element style as the review's `.vcap`.
+- **Each side's caption** shows under its stage in `.cmp-cap`: the review's
+  `.vcap` type (size, leading, colour, pre-wrap), clamped to 20vh rather than
+  the review's 30vh, with its own scroll. Two sides share the height.
 
 ## Invariants
 
@@ -261,7 +265,8 @@ assumptions:
   would apply a stored 1:1, which is itself a script): two Fit stages. Both
   sides' flag forms are present, so choosing which side to flag needs no
   picker; per-side and linked step
-  links; filmstrip links that set B; flag forms that land back on the same pair.
+  links; filmstrip links that replace the URL's active side (B by default,
+  C2); flag forms that land back on the same pair.
   Nothing judgment-bearing hides behind a script.
 - **INV-5 — one record of the stage mode**, shared with the review:
   `stage-one` on `<html>`. Storage never raises.