Charts: shortest water routes between nurseries (precomputed GeoJSON) for move lines and animation #183

What. Replace the straight lines between nurseries with the shortest path by water that never crosses land, precomputed as GeoJSON by a script. Then use each route's length for distances and for the timing of the move animation.

Why. Straight lines cut across the island (for example Haapiti ↔ the east-coast nurseries), so both the map and the "km" figures on /recaptures ("Longest moves between nurseries") mislead. A pup has to swim around the island. A route by water is still a minimum distance, not the path actually swum.

1. Script: scripts/ocean-routes.mjs (e.g. pnpm routes)

• Coastline: OpenStreetMap coastline for Moorea (natural=coastline; src/lib/moorea-outline.json is a simplified copy and may be too coarse near the shore). Credit OSM (ODbL) wherever routes are shown.
• Graph: rasterise the sea around Moorea (≈ 20–50 m cells, out to a few km beyond the reef), or build a visibility graph on buffered coastline vertices. Edges may not touch land; buffer the coast by a few metres so routes don't graze the beach.
• Shortest path: Dijkstra or A* between every pair of nurseries (all ~12 sites, so ~66 pairs), then simplify (string-pulling or Douglas–Peucker) without reintroducing land crossings.
• Output: a FeatureCollection of LineStrings, one per unordered nursery pair, with properties: { from, to, km, straightKm }, plus a test that no route intersects the coastline polygon.
• Deterministic output: same input, same file. Document it in AGENTS.md (Repository map + Commands).

2. Where the GeoJSON lives (decision needed)

Nursery coordinates come from the GPS workbook in data/, which is never committed (golden rule 2). The site already shows the nursery points publicly, but committing a file of routes between them puts those coordinates in the public repo. Options:

• a) run the script during import (DATA_DIR → cache/, in the fingerprint), so nothing derived from the sheets is committed; or
• b) commit external/nursery-routes.geojson if the lab agrees the nursery coordinates can be public (they already are on the map).

Default to (a) unless the lab says (b) is fine. Bump IMPORTER_VERSION if it runs in the importer.

3. Use the routes

• InteractiveMap / MooreaMap draw a move with its route geometry (fallback: straight line if no route).
• /recaptures: "Longest moves" and the pair popups report km by water (straight-line km in brackets or in the CSV).
• Animation timing (the Play on each shark's page, Charts: map shark moves between nurseries and each shark's capture sites #173): the dot moves along the route's geometry instead of in a straight line, and how long each step takes scales with route length (for example a fixed km per second, with a minimum and maximum so very short or very long hops stay readable). Under reduced motion it jumps to the end, as now.
• Optional: on the shark page caption, add "≥ N km by water in D days".

Acceptance

• No route crosses land or the coastline (tested).
• Every nursery pair has a route; distances are plausible against the straight-line km (never shorter).
• Moves animate along the water route, with timing proportional to distance.
• Copy says routes are the shortest path by water, not the path swum.
• CSP unchanged (no new hosts; OSM data is fetched only by the script, never in the browser).

Related: #173 (PRs #178 and #179), #182 (direction arrows should follow the route). Epic: #70