Bundle Station Master v0.5.2, and fix the package version scheme

Submodule pin moves to v0.5.2, which carries the working splash-page
multiplayer door and the GET /api/health probe behind it.

The interface goes back to serving `/` — the splash, which is the front
door to all three ways to play. It pointed at /play.html only because the
splash's multiplayer door was hardcoded to "Coming soon", making the front
page a dead end on a real server; v0.5.2 removes the reason for the
workaround. The splash now probes /api/health on load, so served from here
it opens the lobby and served as a plain static site it says plainly that
it needs a server.

The package version was wrong and is corrected: 1.0.0:0 -> 0.5.2:0. That
1.0.0 was the scaffold's placeholder, left in by mistake — versions.md's
consistency checklist requires the upstream half to match the bundled tag,
which is what makes "bump the pin, bump the version" one mechanical action
rather than a judgement call. No historical version file is needed: the
placeholder carried no migration and was never published anywhere.

The correction is a downgrade in ExVer terms, so StartOS refused to install
over the existing copy ("uninit target range `!` is unsatisfiable"). The
installed package was therefore removed and reinstalled — checked first
that the volume held only an auto-generated join secret and no games. Two
consequences, both one-time and both recorded in UPDATING.md: the join
secret is regenerated, and the host's public domain binding was dropped
with the uninstall and has to be re-added.

Verified on phoenix.local: builds as v0.5.2:0, installs, starts, and serves
/api/health 200 with the right body; the splash carries all three doors
including a live ./play.html?lobby, no "Coming soon" anywhere, all three
ids the probe addresses present, and #lobby rendering on play.html?lobby.

UPDATING.md

@@ -29,10 +29,16 @@ cd station-master && git fetch --tags && git checkout v<new version>
 cd .. && git add station-master
 ```
 
-Then bump this package's own version in `startos/versions/current.ts` (see
-[Versions](../start-technologies/projects/start-sdk/docs/src/versions.md)) with release notes
-naming the Station Master version now bundled — e.g. "Bundles Station Master v0.5.2." — and
-rebuild (`make`).
+Then set this package's version in `startos/versions/current.ts` to **the same number**, as
+`<tag>:0` — `versions.md`'s consistency checklist requires the upstream half to match the
+submodule tag exactly, so v0.5.3 becomes `0.5.3:0`. Give it release notes naming what changed for
+a player, then rebuild (`make`). Raise the `:0` instead, leaving the upstream half alone, when the
+change is packaging-only and the bundled game has not moved.
+
+> The initial release shipped as `1.0.0:0` — the scaffold's placeholder, left in by mistake while
+> the game was at 0.5.1. It was corrected to `0.5.2:0` before the package was ever published
+> anywhere, which is the only reason a downgrade was harmless: `0.5.2:0` sorts *below* `1.0.0:0`,
+> so an installed copy had to be removed rather than updated over.
 
 If the jump carries a multiplayer protocol or persistence-format change (check the game repo's
 `CHANGELOG.md` and `docs/architecture/multiplayer.md`), read it before bumping: a running