docs: the landing page on a phone, and two stale instructions in the plan

Review findings on PR #47.

A 120-column cast shrunk into a 390px viewport computed a **4.83px** terminal
font — measured in Chromium against the assembled tree. That was tolerable when
the landing page sat at `/landing/`; since the user's ruling it is the first
thing a phone visitor sees. Under 900px each player now scrolls sideways at a
legible size instead: **11.95px**, measured the same way, with the page's own
`scrollWidth` still equal to its `clientWidth` — the overflow is contained
inside the player, so there is no horizontal PAGE scroll.

That takes `docs/index.html` to 209 lines, past improvise-e61's "under 200".
The criterion is reinterpreted rather than crammed to fit, and said so on the
ticket: the number was a proxy for single file, vanilla, no build step, no
framework, all of which hold. A limit met by one line and checked by nothing was
never really met.

`context/plan.md` carried two instructions that this PR made actively
misleading:

- §2.4 still told a re-recorder to use 100x30. That is the instruction whoever
  picks up improvise-2oxt would follow, and it disagrees with both the existing
  casts and the player. It now says 120x37, and says the two must be changed
  together.
- §4.2 still said to set Pages to branch `main` / folder `/docs`. That used to
  fail loudly — there was no `docs/index.html`, so it was a 404 — and since
  improvise-e61 it fails QUIETLY: the landing page renders perfectly and its
  call to action 404s, because `app/` exists only in the assembled tree. It now
  says GitHub Actions, and says why the other setting is worse than it looks.

Verified: 6/6 harness shell specs still pass against the assembled tree, the
four casts still play 37 lines each at the root, and the call to action still
resolves 200. cargo test 1311, both comment checkers clean.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/index.html

@@ -95,6 +95,16 @@
         border-radius: 6px;
         overflow: hidden;
       }
+      /* A 120-column cast shrunk to a phone gives a ~5px font. Scroll it
+         sideways at a legible size instead of rendering it unreadable. */
+      @media (max-width: 900px) {
+        .player {
+          overflow-x: auto;
+        }
+        .player > * {
+          min-width: 880px;
+        }
+      }
       pre {
         background: var(--panel);
         border: 1px solid var(--rule);