fix(r3): fold heid's bug hunt — no link offers a pair that 404s, NUL booth names, a FIFO marker, encoded view-state names

Navigation was built from the review ring while the compare GET also demands
containment, so an outside symlink (which stays in the ring) was offered by
the strip, the steps, the review's Compare control and the flag landing, and
404ed on arrival. Every one is now built from the compare ring (the review
ring filtered by the same conjunction, _in_booth).

Two pre-existing gaps compare inherits, fixed at the source: resolve_booth
caught only OSError, so a NUL in the booth segment was a 500; record_view
opened its marker blocking, so a planted FIFO hung every look. Plus: the page
treats %73ide=a as side=a, and the subgrid engine floor is stated. Two
findings refuted (a chorded click mid-drag never fires pointerup, measured;
booth_items never yields an unquotable rel). r3.toml: 57 rows.

tests/test_compare_browser.py

@@ -419,6 +419,22 @@ def test_a_save_keeps_the_active_side(browser, live):
     assert picked == ("m-2-forge-s23.png", "r-1-dock-s11.png") and side == "a", (picked, side)
 
 
+def test_an_encoded_view_state_name_is_still_view_state(browser, live):
+    """`%73ide=a` IS `side=a` to the server, so the page must treat it as view
+    state too: after X, a reload shows the side X chose, not the stale one
+    the encoded parameter still named (heid bug hunt, hulda)."""
+    base, root = live
+    _bakeoff(root)
+    page = browser.new_page(viewport={"width": 1440, "height": 900})
+    _open(page, f"{base}/b/g/compare?a=m-1-dock-s11.png&b=r-1-dock-s11.png&%73ide=a")
+    first = page.evaluate("document.querySelector('.cmp-side.is-active').dataset.side")
+    page.keyboard.press("x")
+    page.reload(wait_until="networkidle")
+    after = page.evaluate("document.querySelector('.cmp-side.is-active').dataset.side")
+    page.close()
+    assert (first, after) == ("a", "b"), (first, after)
+
+
 def test_without_js_every_judgment_and_step_still_works(browser, live):
     """INV-4. Scripts off: the pair renders in two Fit stages, the step and
     strip links navigate, both flag forms are there and a flag lands back on