v0.42.1

The tool now says the version it was released as

v0.42.0 was tagged on a tree whose VERSION still read 0.41.2, so the
published executable announces the previous release: --version says
0.41.2, and the browser GUI prints "Loaded lightwebpres v0.41.2" for a
0.42.0 vendor. Nothing else differs — a series built with v0.42.0 and
rebuilt with this one produces byte-identical pages.

A guard now compares the tool against the release rather than against
itself: if HEAD is the newest tag, VERSION must equal it; if HEAD is
ahead, VERSION must be strictly greater. It cannot fire before a tag
exists, and fires on the first run after it.

A link to the series-navigation card lands again

The alias that keeps pre-0.42 links working was the card's rank, and for
every card but one that was right. A series-nav's id was sN-series —
the name written into the address bar when a reader arrived there, and
the name its own pagination dot linked to. v0.42.0 emitted only sN for
it, so #s3-series pointed at nothing in a rebuilt series.

Both names are emitted for that type now, and only for that type. Found
by rehearsing the upgrade — building a demo with v0.41.1, rebuilding it
with this version, and looking for a landing place for every id the first
build had.

Migration

From v0.42.0: rebuild, and the series-navigation anchor works again.
Nothing else changes.

From v0.41.x: as for v0.42.0 — nothing to do, old links keep working
through the alias.