Make line-block toggles (lists/blockquote/code fence) undoable (#20)

[Wavesonics]

Problem

Toggling a numbered/bulleted list, blockquote, or code fence was not undoable (#20). Pressing Ctrl+Z after toggling a list did not remove the list — it reverted the previous edit.

The toggles in MarkdownExtension (toggleOrderedList/toggleBulletList/toggleBlockquote/toggleCodeFence → toggleLineBlock) called applyLineBlock/demoteLineBlock, which mutated both the line's paragraph indent (via updateLine) and a line-anchored rich span (via richSpanManager). Neither went through editManager.recordEdit, so the change was invisible to undo/redo. PR #26 made standalone rich spans undoable but deliberately left line-block toggles out; this PR finishes that work.

Design: dedicated TextEditOperation.LineBlock variant

I chose a dedicated variant over a generic Composite transaction. The line-block toggle has a fixed, well-defined shape, so a dedicated op lets us capture each affected line's prior/next AnnotatedString and block-span set directly and invert cleanly — simpler and less error-prone than threading a list of sub-operations each with their own metadata. transformOffset is a no-op because toggles change paragraph style only, not text length.

Each LineBlockChange snapshots, per affected line:

• contentBefore / contentAfter — exact line content (so undo restores paragraph + text style precisely)
• blockSpansBefore / blockSpansAfter — the line-anchored block span styles present in each state

Capturing the full before/after span set (not just the toggled style) means a mutually-excluded block demoted as a side effect (e.g. toggling an ordered list onto a bullet line) is also restored correctly in the same atomic step.

MarkdownExtension.toggleLineBlock now routes through editManager.toggleLineBlock, which captures before/after, applies the toggle via the direct (non-recording) path, then records exactly one LineBlock history entry. The inner span mutations deliberately use the direct richSpanManager path to avoid double-recording. importMarkdown is unchanged and stays on the direct path, so document loading still does not pollute undo history.

Exhaustiveness

Every exhaustive when (operation) over TextEditOperation was extended:

• TextEditManager — applyOperation, undo (new applyLineBlockState / undoLineBlock; redo reuses applyOperation)
• RichSpanManager.updateSpans — passthrough (handleSpanOnly), toggles don't move text
• SpellCheckState.invalidateSpellCheckSpans — null, no text change
• SpellCheckingTextEditor.computeAffectedRanges already had an else branch.

Verified with the full ./gradlew check (the cross-module build that caught #26's first break).

Tests

LineBlockUndoTests and LineBlockRedoTests, all driving the real markdown.toggle* entry points:

• ordered list undo/redo restores both span and paragraph indent
• the reported scenario: type a char, toggle a list, undo → list fully gone, char intact; redo → list back, char intact
• blockquote and code fence undo/redo
• multi-line list toggle undo/redo
• demote (toggle-off) is itself an atomic undoable entry

Verification

./gradlew check → BUILD SUCCESSFUL (iOS tasks skipped on Windows).

Fixes #20