Filter by more than one library

[abernier]

The rail's library filter held one label at a time, so the only way to
ask for two was to type them into the search box and hope the words
turned up somewhere in the haystack.

The Select is multiple now: picking adds, picking again removes, and
the popup stays open across the whole run rather than closing on every
choice. alignItemWithTrigger goes off with it — there is no single
selected row for the popup to slide under once there are three.

One rule for several picks

An example has to carry all of them, the same rule the search terms
already follow. Every pick narrows, so the count under the field only
ever falls, and Cannon + Rapier — two physics engines that never
share an example — comes out empty and says so through the Empty
block that was already there.

The URL

?library= keeps its name and its labels and becomes a comma-separated
list. A link written when the field took a single value parses as the
one-element list it always was, so there is nothing to migrate,
deprecate or delete. bootNav is untouched for the same reason: it asks
whether the param is there, not what is in it. An emptied list drops the
param outright — nuqs compares arrays by their items, so [] is the
default it clears on.

The trigger, and the row that left

One line at every count the rail can hold: the first pick spelled out
and the rest as a tally, Drei +2. Not a chips field — at 200px it
would grow a line per library, and the pre-paint skeleton standing in
front of it on a filtered arrival can only ever be one line tall.

The All libraries row is gone from the popup. In a list where every
row is a checkbox it was the one that was not, and it says nothing that
unpicking the last library does not already say. Empty is now inferred:
no picks, and the trigger reads All libraries in the placeholder tone.
Clear filters in the empty state still drops everything at once.

Checked by hand

tsc --noEmit, eslint, prettier --check.

In the browser: ?library=Drei → 156, ?library=Drei,Rapier → 11 with
the rail revealed, ?library=Cannon,Rapier → 0 and the empty block,
picking and unpicking down to nothing, and the card links carrying the
list into an example.

🤖 Generated with Claude Code