Clip.summary splits the whole clip into lines to show its first one, on every row of every panel redraw

## What happens

`Clip.summary` (`Sources/UttrflowClipboard/Clip.swift:115-120`) is the one line a row shows:

```swift
text.split(whereSeparator: \.isNewline)
.first
.map(String.init)?
.trimmingCharacters(in: .whitespaces) ?? ""
```

`split` walks the whole clip and builds every line to keep only the first. `PanelPresenter.row(for:in:isSelected:)` reads it for every listed row on every keystroke (`Sources/UttrflowUX/PanelPresentation.swift:371`). The result is also not capped. A single-line clip, such as minified code or a log line, becomes the row's `Text`, its tooltip (`QuickPanelView.swift:424`) and its VoiceOver label (`QuickPanelRowAppearance.swift:43`) at full length, up to the 2 MB clip bound.

## Measured

Throwaway Release harness, Apple silicon:

- `summary` over 916 clips of 8 to 16 KB each: **116 ms**. `PanelPresenter.present` after ↓ on the same list: 116 ms, so almost all of each arrow key is this.
- `summary` on one 1.8 MB multi-line clip: 19 ms, every redraw.

## Why it matters

A full history (the copied pool holds up to 8 MB) makes every arrow key and every search letter lag. A VoiceOver user who lands on a long one-line clip hears all of it.

## Acceptance criteria

- `summary` reads only as far as the first line break, or a fixed number of characters if that comes first, whichever is sooner. It never visits the rest of the clip.
- The summary is capped at a length a 420-point row can use (a few hundred characters). Truncation stays the view's job.
- Tests in `Tests/UttrflowClipboardTests/ClipTests.swift`: the first line is unchanged for ordinary clips; a clip whose first line is longer than the cap comes back capped; leading whitespace and CRLF behave as today.

## Where to start

- `Sources/UttrflowClipboard/Clip.swift:114-120`
- Test to extend: `Tests/UttrflowClipboardTests/ClipTests.swift`
- Before pushing, run `make verify` (export `DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer` first). It is the same command CI runs, and it enforces the 95% coverage floor per module.
- Read [CONTRIBUTING.md](https://github.com/uttrflow/uttrflow-swift/blob/main/CONTRIBUTING.md) first, and say on this issue that you are taking it.

**Size:** XS to S, about 1 to 2 hours.

Read CONTRIBUTING.md, then inspect Clip.summary in Sources/UttrflowClipboard/Clip.swift:114-120 and extend Tests/UttrflowClipboardTests/ClipTests.swift. Verify ordinary first lines, the cap, leading whitespace, and CRLF behavior while ensuring reading stops at the first line break or cap. Run make verify with the specified DEVELOPER_DIR; done means the tests pass and summaries are bounded for redraws and accessibility labels.

Written by the indexing model from the issue text.