feat(layers): add shipwrecks layer (EMODnet + Wikidata)

A new Events layer maps shipwrecks with a card per wreck:
- EMODnet Human Activities heritage shipwrecks (about 6,500 in European
  waters, CC BY 4.0), skipping rows taken from wrecksite.eu.
- Wikidata shipwrecks with a Wikipedia article plus sinking events
  (CC0); an event folds into its ship for the date and death toll.
- A same-origin /api/shipwrecks proxy refreshes once a day and serves
  the last snapshot as stale for up to a week.

Markers are a boat icon coloured by era and sized by deaths; four era
chips filter the layer. The card follows the air-accidents layout
(deaths first). Text that names people is dropped; victims appear as
a count only.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/data/layerState.js

@@ -691,6 +691,7 @@ export const LAYER_STATE_REGISTRY = Object.freeze([
     optionOwner: 'satellites',
   }),
   Object.freeze({ id: 'sea-marks', token: '6', disposition: 'enabled-only' }),
+  Object.freeze({ id: 'shipwrecks', token: '9', disposition: 'enabled-only' }),
   Object.freeze({ id: 'ski-pistes', token: '7', disposition: 'enabled-only' }),
   Object.freeze({
     id: 'street-level',