Tracker: replace the category tabs with a scope picker

[BryanFRD]

Reworks the tracker's Blocks / Items / Entities tabs into a single scope field.
Discussed on Discord first.

Rebased on master, two commits. #57 has landed so the meta stamp is out of this diff, the
segmented control from #59 and #73 is used as it stands, and the commands category from
#70 is wired into the scope model.

What changes

• Categories are collapsible groups with a checkbox each, and so is every behavior inside.
  Scope to all of Blocks, or to BedBlock and FireBlock, or mix the two. Picking a whole
  category drops its individual behaviors, in the selection and in the URL.
• Search sits in the same field and matches categories, behavior names and entry names, so
  copper_block finds WeatheringCopperFullBlock. The separate search box is gone.
• Rows link to the .rs file that implements them. The sync script already had the path
  and threw it away.
• List / TODOs is a view toggle rather than a tab: 83 open items across 45 behaviors.
• Scope, view, status and query live in the query string.

Default scope is Blocks, so first load shows 263 classes rather than 559. An absent
scope param means first visit, scope=all means deliberately cleared.

Second commit

SegmentedControl placed its pill from offsetLeft and offsetWidth, which round to
whole pixels: 2.42px of space on the right against 2.00px top and bottom. It measures with
getBoundingClientRect now. Container and pill were both rounded-2xl at different
heights, so both are rounded-full. Upstream styling, separate commit, easy to drop.

Note

implementation-status.json is left as master has it, so the source links stay hidden
until the next nightly sync emits path. Regenerating it here would stamp
meta.steel_commit with whatever my local Steel checkout sits on.

Tested

bun run build clean, 116 pages. tsc --noEmit clean under src/ and scripts/, the
rest is a duplicate satteri under node_modules. Browser check: default scope, tri-state
parents, mixed selections, entry search, URL round-trip, the commands group, pill insets
on all four sides.

[DarkMrMelther]

Fix merge conflicts pls 😸

[ArSiJa8]

Ohh, big pr, im still gonna read through it just for fun though :>

[tn-lorenz]

This could be goated pls fix conflicts

[tn-lorenz]

Will look at this in the evening 👍🏼