feat(diff): add follow-latest selection chip

[justsomelegs]

What Changed

This PR adds a Latest selection to the diff panel that follows the current head checkpoint as new checkpoints arrive.

Changes in this PR:

• add explicit diff route state for Latest selection
• add a Latest chip to the diff panel that stays on the newest checkpoint
• keep individual Turn N chips as pinned checkpoint selections
• group Latest with the current head turn chip in the UI
• preserve the timestamp on the grouped turn chip
• add route parsing coverage
• add browser coverage for selecting Latest and for following newly-arrived checkpoints

Why

The diff panel already lets you inspect specific checkpoint diffs by turn, but it did not have a way to stay on the newest checkpoint as the thread continued to update.

This adds a dedicated Latest mode with clear semantics:

• Latest = follow the newest checkpoint
• Turn N = pin a specific checkpoint diff

That makes it easier to monitor the current head diff without losing the ability to inspect a fixed turn.

UI Changes

This adds a grouped Latest and current-head turn control to the diff panel.

The grouped turn chip still shows the timestamp so it stays consistent with the rest of the turn strip.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

Note

Add follow-latest selection chip to the diff panel

• Adds a diffSelection=latest URL parameter and a 'Latest' chip in DiffPanel.tsx that keeps the diff view tracking the most recent turn as new checkpoints arrive.
• When follow-latest mode is active, diffTurnId and diffFilePath are ignored and the newest turn is always used; selecting a specific turn or 'All turns' exits this mode.
• Extends diffRouteSearch.ts to parse and strip diffSelection, accepting only 'latest' as a valid value.
• Adds browser tests covering chip selection behavior and URL state updates when new checkpoints are appended in follow-latest mode.

^{Macroscope summarized bf71000.}

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