fix(tickets): smooth cursor nav

[StuBehan]

Summary

Holding ↑/↓ on the Tickets tab was jumpy, and crossing a group boundary made
the highlight appear to "bounce back up." This makes cursor navigation track
smoothly through a group's branches and straight on to the next group. The
selection logic was never wrong — this is all rendering/scroll.

Changes

• Snap the scroll instead of animating it (the actual bounce) —
  scrollTo(anchor: .center) was wrapped in a 0.15s animation and fired on
  every key-repeat. On a long list (100+ rows) the animations piled up, the
  scroll lagged the selection, then settled with an upward snap — which read as
  the highlight bouncing up. Dropping the animation pins the selection and
  tracks it smoothly.
• Memoize the group derivation — visibleOutcomeGroups() re-ran the full
  grouping (a regex per ledger record + branch breakdown + sorts) on every
  body render, i.e. every selection change. Now cached and invalidated only
  when the ledger changes (handoffsRevision); the hideShipped filter still
  runs per call.
• Anchor the header scroll to the header row, not the whole group container
  (which spans all its branches) — so scrollTo centers a small, consistent
  target whether you land on a header or a branch.
• Highlight only the selected header row, not the entire group block —
  keeps the selection a consistent small marker at every row.

Testing

• ./build.sh → Build complete, zero warnings.
• Diagnosed with live telemetry (temporary, since removed): the selection index
  moved perfectly monotonically (clean ±1 per keystroke, every index mapping to
  the right row) — which is how we confirmed the "bounce" was scroll/animation,
  not the selection.
• Needs a manual eyeball (the panel can't run in CI): hold ↑/↓ through a
  multi-branch group and across boundaries — tracks smoothly, no bounce.

Related issues

Follow-up to the Tickets-tab repo-grouping work. No tracked issue.

[hiskudin]

Reviewed the diff. Right diagnosis, right fix — no changes needed from me.

• withAnimation(.easeOut(duration: 0.15)) around proxy.scrollTo(anchor: .center) firing on every key-repeat is exactly the bounce cause. Dropping the animation is the only fix that reads right at key-repeat cadence.
• Memoization is correct: handoffsRevision didSet clears cachedOutcomeGroups, so ledger changes bust the cache; the hideShippedTickets filter is still applied per call on top, so toggling "hide shipped" or a PR flipping to shipped both still take effect without a ledger tick.
• visibleOutcomeGroups() is only ever called from SwiftUI body renders + keyboard indexing (both main-thread), so the cache read/write stay serialized without an explicit lock. Consistent with the rest of PanelNav.
• The .background moving from the whole groupRow container to just the header HStack fixes the "entire group lights up when a header is selected" perception cleanly, and .id(Self.headerID(group)) moving inside the header HStack is fine because that HStack is unconditionally rendered — proxy.scrollTo(id:) still finds it.

Ship it.