Test note checkboxes written with single-quoted or unquoted type, and NotePromotion escaping quotes

## What happens

`Sources/UttrflowClipboard/NoteChecklist.swift`:

- lines 68-70: `inputBox` accepts `type='checkbox'` and `type=checkbox`, but tests only use `type="checkbox"`. The `else { return nil }` for an `` that isn't a checkbox has never run either;
- lines 88-89: `hasAttribute` finding `checked` followed by `/`, whitespace, or the end of the tag;
- line 123: `tokens(of:in:)` with a `class="` that never closes;
- lines 137-139 and 149: `replacingToken` and `withoutCheckedAttribute` finding no spelling to change;
- lines 172-173: `NotePromotion.escaped` turning `"` into `"` and `'` into `'`.

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

Ticking a box in a note rewrites HTML that other apps and websites wrote, and different editors write checkboxes differently. If single quotes stop matching, a box can't be ticked. If escaping `'` or `"` breaks, promoting a plain clip such as `it's "done"` to a note garbles the text when it is read back.

## 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/NoteChecklist.swift` and look for the `0` counts on the lines named above.

## Acceptance criteria

- Table cases: ``, ``, and `` (not a box).
- `NotePromotion.note(from: "it's \"done\"")` contains `'` and `"`, and reads back through `RichTextPlainForm.plainText(fromHTML:)` as the original.
- An `

Read CONTRIBUTING.md, then open Tests/UttrflowClipboardTests/NoteChecklistTests.swift and the “E5 · ticking a box in a note” suite. Add the specified table and escaping cases, run the suite with swift test --filter, verify each test fails when its covered branch is temporarily broken, and finish with make verify.

Written by the indexing model from the issue text.