Footnote Duplicate Definition Normalization Runtime

Goal

Implement duplicate-definition normalization for footnotes so the first definition stays canonical, later duplicates become explicit invalid siblings, and the user gets a repair path instead of only a warning.

Scope

@platejs/footnote package queries / transforms / API
app footnote UI in apps/www/src/registry/ui/footnote-node.tsx
docs / release note sync if runtime behavior changes

Current Findings

The registry already sorts definitions by path and definition() already returns the first one.
Duplicate detection already exists through:
- api.footnote.hasDuplicateDefinitions
- api.footnote.duplicateIdentifiers
- api.footnote.definitions
The current app UI warns about duplicate references in hover preview, but duplicate definition blocks still behave too much like valid definitions.
Backlink navigation from a later duplicate is misleading because references belong to the canonical first definition.

Working Plan

add package-level tests for canonical-first duplicate semantics and a repair transform
add app-level tests for invalid duplicate definition UI and repair action
implement package helpers / transform
implement app duplicate definition UI
verify with tests, build, typecheck, lint, browser

Progress Log

2026-04-06: read current registry, package API, app UI, and the deferred spec note
2026-04-06: added package helpers for duplicate definitions plus tf.footnote.normalizeDuplicateDefinition
2026-04-06: changed duplicate definition blocks so later duplicates render as invalid siblings and expose a renumber repair action instead of backlink behavior
2026-04-06: synced docs, changeset, and a reusable learning