feat(viewer): measure shortest route along the cave

[MattNotarangelo]

Background

• The measure tool answered "how far as the crow flies?" but cavers usually need "how far through the cave?" — for trip planning, rigging, and rescue pre-plans. This adds that as a natural extension of the existing two-station measure flow.

What Has Changed

• New pure module src/viewer/route.ts: shortest path between two stations via single-source Dijkstra with predecessor tracking over the non-splay leg graph, early-exiting at the target. ~18 ms on the 22k-station Migovec model.
• Measure panel gains an "Along cave" row (or "no route" when the stations aren't connected); the route is highlighted in the view as a fat cyan line drawn over the centreline (a thin line was invisible underneath the fat centreline), redrawn on vertical-exaggeration changes and disposed with the measurement.
• Equate handling (important find): Therion .lox writes an equated station once per survey — different ids/names, identical coordinates, no joining shot. The Migovec example therefore fragments into ~600 centreline pieces, and most routes initially came back "no route". The route graph now treats coordinate-coincident stations (0.1 mm key) as one node, which fuses Migovec into a single 7,725-station connected component. Verified: a pair that previously failed now reports 424.9 m along the cave vs 134.1 m straight line.
• Tests: 7 cases in test/route.test.ts (chain summing, loop shortest-way with reversed leg orientation, splay exclusion, disconnection, equate connectivity, self-route, bad ids). 71/71 passing.
• README: measure-tool docs updated.

Note for a follow-up

The same .lox fragmentation affects the distance-from-entrance colour mode (entranceDistances in coloring.ts) — large parts of .lox models colour grey/unreachable today. Deliberately not fixed here to avoid conflicting with #2; worth the same canonicalization in a follow-up.

Screenshots/Video

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Checklist

• Self-review completed
• Tests added or updated
• Tested locally

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View logs

[MattNotarangelo]

Testing feedback fix: clicks near the centreline often landed on invisible Therion "anonymous" wall points (stations named ./- — 13,858 of them in the Migovec example) which hang off splay-only legs, so the panel said no route. The .lox parser now flags them anonymous (unpickable), matching Therion's naming convention; pickable stations drop to the 8,294 real ones and a 34-pair sweep across the system finds a route every time. Genuinely-named stations with a trailing dot (S1.9. in the golden fixture) stay named. 73/73 tests passing.

[MattNotarangelo]

Second testing-feedback fix: distance-from-entrance colouring suffered the same .lox equate fragmentation as routing — only 2.3% of the Migovec example was reachable from its entrances (the rest coloured grey). entranceDistances now canonicalizes coordinate-coincident stations like findRoute does: 100% reachable (8,293/8,294; the one remainder is genuinely isolated), max 3,267 m from the nearest entrance, 41 ms. This closes the follow-up flagged in the PR description. 74/74 tests passing. Note: this touches coloring.ts, which #2 also modifies in different regions — if a conflict appears when the second one merges, I'll rebase it.