[gnapse]

PR #9's two fresh reviewers planted bugs in the layer commands. Four passed every test. This PR adds a test for each. Only src/editor/layers.test.ts changes: 4 tests, 40 lines. No bug found, so no code changed.

New tests

• jumpToItem backward with no last jump (⌥H in the palette passes null). From between items, from an item's start, and from before every item.
• removeAltVariant and removeVersionVariant with the removed variant before the active one. The active text stays on the page, and the saved Markdown marks it.
• A swap is its own undo step. Type next to an alternative, cycle it, and undo once: only the swap goes. The next undo takes the typing.

Each test fails against its planted bug

Checked on this branch's base, main 2e9a7e8. With no plant, 46/46 pass. Each plant fails exactly one test.

Planted bug in commands.ts	Failed test	Got, expected
findLastIndex((i) => i.from < head) → findIndex((i) => i.from > head)	jumps back from the caret with no last jump	[35, 1], [6, 0]
same, < → <=	jumps back from the caret with no last jump	[6, 0], [35, 2]
removeVersionVariant keeps active	keeps the active version when one before it is removed	'Three.', 'Two.'
removeAltVariant keeps alt.active	keeps the active variant when one before it is removed	'the strain here', 'the pressure here'
annotations: ownStep removed from replaceAlt	undoes a swap on its own, not with the typing next to it	first undo also removed " here"

Left as is

The old test "cycles the word at the caret before its paragraph" puts the word in a paragraph with no versions, so it can't check which one wins. PR #9's "cycles both ways at the caret" already does: making cycleAtCaret try the paragraph first fails it ('First.', expected 'Second one here.'). The old test is unchanged. Its name still claims more than it checks; a rename would fix that.

Checks

pnpm verify passed (types, tests, web build, Rust format, lint, tests). Nothing here touches the app window, AI calls, or files.