Test that BackedSpeechEngine refuses silent audio, and audio whose speech is too short once silence is trimmed, with nothingHeard

## What happens

`Sources/UttrflowPipeline/PieceJoiner.swift` joins the pieces of a long dictation. These early returns have never run:

- line 72: a piece that is empty or only whitespace is skipped;
- line 76: when only one non-empty piece remains, its text is returned without any list or paragraph layout;
- line 109: a restatement check at the very first word;
- lines 158 and 161: `itemise` finding no body after the sequence word;
- line 176: `sequence` when a piece ends on the prefix ("number" or "point") with nothing after it;
- line 205: `opensTopic` for a word no longer present.

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

Long dictations are cut into pieces at pauses, and a pause can produce an empty piece. If the empty-piece skip or the single-piece early return is broken, the joined text can gain a stray bullet, a blank paragraph, or lose a word at a seam. All of that lands in the user's document.

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

## Acceptance criteria

- `laidOut(["", "first buy milk", " "], under: )` returns the one piece's text unchanged, with no bullet.
- `laidOut(["first fix the build", "number"], ...)` does not crash, and keeps "number" as a word.
- Two pieces where the second is only a sequence word ("first buy milk", "second") do not make an empty list item.
- 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/UttrflowPipelineTests/PieceJoinerTests.swift`. It already builds `DestinationFormatter` values with and without `.lists` and `.paragraphs`.

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.

Read CONTRIBUTING.md, then open Tests/UttrflowPipelineTests/PieceJoinerTests.swift and the named branches in Sources/UttrflowPipeline/PieceJoiner.swift. Add tests for empty pieces, single-piece layout, restatement, empty itemise bodies, sequence prefixes, and missing-topic words; run the filtered suite and confirm each test fails when its branch is temporarily broken, then run make verify.

Written by the indexing model from the issue text.