fix: four defects the U4 bug-hunt panel found in code it did not add

All four pre-date U4 and sit in files it touched, which is why a diff-scoped
robustness lens saw them. They are separated from the unit's own commit so the
feature history stays readable; the release tags both.

* A booth name reached a JS string context. The confirm dialogs interpolated
  the name into a string literal inside `onsubmit`. Jinja's autoescape is
  HTML-attribute escaping, not JS-string escaping: the browser decodes the
  entity back to a quote before the JS parser sees it, so a name crafted to
  close the string executed on submit. Booth names are agent-authored — making
  a folder under the data dir is the whole API — so this was a live path, not a
  theoretical one. The name now travels as a data attribute to a delegated
  handler, where escaping is escaping.

* An unreadable `links.md` returned 500 for the whole booth page. `is_file()`
  then an unguarded `read_text()`. The board is one tile on that page, and a
  page that will not load is worse than one missing a tile — the posture
  `read_blurred`, `marks_for` and `read_manifest` already take.

* The index order had no tie-breaker, which violates the deterministic-order
  invariant. Equal-mtime booths fell back to whatever `iterdir()` yielded, and
  two booths landed by one `rsync` batch share an mtime exactly. Now
  `(mtime, name)` reverse: newest first, then name. The operator refers to
  cards positionally, so a sequence that moves between renders misfiles his
  judgment rather than crashing.

* `/b/<n>/marks.json` reported damage as empty success. `booth marks` exits 3
  on an unreadable file precisely so a caller can tell "not yet" from "broken";
  the HTTP mirror — the only reader a remote session has — returned the same
  empty list for both. It now carries `error` and `detail`. The status stays
  200 deliberately: reads are lenient here, and a pinned status code is a
  promise to remote clients this fix has no business breaking.

Each has a regression test. 410 tests.

ROADMAP.md

@@ -1,7 +1,7 @@
 # The Booth — roadmap
 
 Design: [`docs/design/information-architecture.md`](docs/design/information-architecture.md).
-Current version: `0.3.0` (U1, U2, U4 and U5 landed; extracted from eshpfi 2026-09-21).
+Current version: `0.4.0` (U1, U2, U4 and U5 landed; extracted from eshpfi 2026-09-21).
 
 ## v1 target
 
@@ -13,7 +13,7 @@ defect — not a wish. The measurements are in the IA doc.
 | 1 | ~~**One item record**~~ — **landed `ce598b3`** | captions never reach the zoom view (never sent, not lost) | U1 |
 | 2 | ~~**Marks**~~ — **landed `c7f9437`, released `v0.2.0`** | 5 mechanisms for 1 job; operator→session loop runs through chat | U2 |
 | 3 | **Declared embed seam** — `/_booth/embed.js`, chrome mounts via DOM | 6 regexes injected into arbitrary author HTML, load-bearing for asks | U3 |
-| 4 | ~~**Derived lifetime**~~ — **landed** | 70% of booths on the `.forever` escape hatch (54% when first counted) | U4 |
+| 4 | ~~**Derived lifetime**~~ — **landed `c3a97c1`, released `v0.4.0`** | 70% of booths on the `.forever` escape hatch (54% when first counted) | U4 |
 | 5 | ~~**Self-announcing booths**~~ — **landed `c015a91`, released `v0.3.0`** | job 5 had no home, so it lived on the link board as 145 dead rows | U5 |
 | 6 | **Benches** — registry, identity, enforced rule, migration | 69% link-board rot; the same bench posted 5× | U6 |
 | 7 | **Navigation at 270 items** — sections, rail, filters, grid keyboard | one flat wall; subfolder structure discarded at render | U7 |