feat(input): list commands, keyboard editing, and empty-item anchors (lists 4/5)

[wildseansy]

The list stack (per the review ask, now including ordered lists — the shared machinery made them cheap to add):

1. refactor(input): generalize empty-line anchor machinery beyond headings (lists 1/5) #510 — anchor machinery refactor (no behavior change)
2. feat(input): list block types + marker rendering (lists 2/5) #511 — list block types + marker rendering
3. feat(input): parse and serialize lists (lists 3/5) #512 — parser + serializer
4. feat(input): list commands, keyboard editing, and empty-item anchors (lists 4/5) #513 — commands + keyboard + ZWSP/empty anchors
5. feat(input): list state, toolbar, docs, and e2e (lists 5/5) #514 — state + toolbar + docs + e2e

GitHub can't base a fork PR on another fork branch, so every PR targets main and the diff shown for PR N is cumulative — this PR's own change is its last commit (each stage is exactly one commit on the previous branch). We'll merge bottom-up; after each merge the next PR's diff collapses to just its own commit. Checkout feat/list-5-state to test the full feature end-to-end.

This PR (last commit) — the tricky one

• Commands: toggleUnorderedList / toggleOrderedList / indentList / outdentList. Indent preserves the item's list type and clamps to the previous item's depth + 1; outdent at depth 0 removes the marker; toggling one list type over the other replaces it.
• Enter is handler-driven via continuesOnNewline (default false): a non-empty item continues as a same-type sibling at the same depth, an empty item exits the list. Headings keep reporting false — no per-type branching in the orchestrators.
• Keyboard: Tab indents (hardware + software), Backspace at an item's start outdents then un-lists at depth 0, including at the document start.
• Android ZWSP anchor: a LeadingMarginSpan applies neither indent nor marker to an empty paragraph, so an empty list line is anchored by a zero-width space — settled before span stamping (one marker per line), re-entrancy-guarded, stripped the moment the line gains content or stops being a list item, and stripped by the serializer so it never reaches markdown. iOS needs no counterpart: the paragraph style carries the indent and the layout manager draws the empty-line marker explicitly.
• Renumbering: edits that change the line count re-stamp the whole document so ordered ordinals and the depth clamp propagate past the edited line; same-line typing keeps the scoped per-keystroke path.
• External plain-text paste parses as markdown (closes the paste gap flagged in feat(input): heading blocks (H1-H6) on the block pipeline #497); Android's "Paste as plain text" stays literal.

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