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/voice/layerManifest.js

@@ -457,6 +457,8 @@ export const VOICE_OFF_LAYERS = Object.freeze({
   // query for this layer wait until there is room.
   'drone-airspace':
     'informational airspace layer; no room in the prompt budget',
+  // Same budget: a historical wreck archive waits for room too.
+  shipwrecks: 'historical shipwreck archive; no room in the prompt budget',
 });
 
 const TYPE_NAMES = Object.freeze({