refactor(input): generalize empty-line anchor machinery beyond headings (lists 1/5)

[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

Introduces BlockType.ANCHORED (Android) and ENRMBlockTypePersistsWhenEmpty (iOS) and routes the block store's adjust/normalize passes, the formatter's zero-length anchor stamping, and the views' orphan-anchor prune through them instead of heading-only checks. Pure refactor — headings are the only anchored type until #511 adds the list types.

Verified: ktlint + compileDebugKotlin green, TS green, example iOS build green (whole stack).

🤖 Generated with Claude Code