Test that the clipboard watcher keeps a copy that has only HTML on the pasteboard, using its plain form

## What happens

`Sources/UttrflowClipboard/PasteboardWatcher.swift:120`: when a copy has no plain-text flavour but does have HTML (some web editors and mail composers put only HTML on the pasteboard), `newClip(at:)` derives the text with `RichTextPlainForm.plainText(fromHTML:)`. That `html.map(...)` has never run. Every watcher test writes plain text, either alone or next to HTML.

Also never run: `NoticedClip ==` comparing two clips' pictures (lines 190-193).

Measured on origin/main `26d7bc1` with `swift test --enable-code-coverage` (5,384 tests, all passing) and `xcrun llvm-cov export` over the test binary. The regions named below have an execution count of 0: no test runs them, so a change that breaks them passes `make verify`.

## Why it matters

Without this branch, copying from an HTML-only source records nothing, and the user's copy is missing from clipboard history with no explanation.

## How to reproduce

Run `swift test --enable-code-coverage`, then `xcrun llvm-cov show -instr-profile .build/debug/codecov/default.profdata .build/debug/UttrflowPackageTests.xctest/Contents/MacOS/UttrflowPackageTests Sources/UttrflowClipboard/PasteboardWatcher.swift` and look for the `0` counts on the lines named above.

## Acceptance criteria

- A test calls `clipboard.write(nil, html: "

Hello world

")`. `newClip(at:)` should return a clip whose `text` is "Hello world" and whose rich text is kept.
- A test writes HTML-only content that is blank once plain (`

`) and expects `nil`.
- Two `NoticedClip` values with the same clip but pictures of different sizes are not equal.
- Each new test fails when the branch it covers is broken. Check this by hand before opening the PR: temporarily change the branch (flip the condition or return the other value), see the test fail, then restore it.

## Where to start

- `Tests/UttrflowClipboardTests/PasteboardWatcherTests.swift`. `FakeClipboard.write(_:html:picture:from:marked:)` already accepts `nil` text.

Read `CONTRIBUTING.md` first. Iterate with `swift test --filter `, then run `make verify` before pushing. Only tests change; no product code needs to.

**Size:** S, under an hour.

Read CONTRIBUTING.md, then open Tests/UttrflowClipboardTests/PasteboardWatcherTests.swift and use FakeClipboard.write(_:html:picture:from:marked:). Add coverage for HTML-only plain-text extraction, blank HTML, and NoticedClip picture equality; run the filtered suite and make verify. Done means each test fails when its covered branch is temporarily broken.

Written by the indexing model from the issue text.