fix: build !path geojson.io link as a base64 data URI

[sg]

Problem

!path map links still failed on geojson.io ("unterminated string in the json data") even after the slash-encoding fix (#19) — including paths with no / in any name.

Diagnosis

Resolving a failing da.gd link showed the data arrives intact and parses as valid, complete JSON — so it's geojson.io's rewritten (early-2026) loader mishandling our percent-encoded inline #data=data:application/json,… form, not our data. Confirmed by testing the exact failing payload in several formats:

• A — current percent-encoded inline → fails
• B — data:application/json;base64,<b64> → works
• C — bare base64 → invalid URL
• D — a tiny inline payload → works

A fails while D (tiny) works ⇒ a size/encoding-bloat problem with the inline form (percent-encoding inflates the payload ~3×). Base64 (B) is reliable and ~20% shorter.

Fix

_geojson_io_url now base64-encodes the GeoJSON into a ;base64, data URI. Dropped the unused urllib.parse import; added base64. This also subsumes the earlier slash issue — base64 has no characters that need escaping in a fragment.

Verification

New format round-trips to valid JSON through base64 and through da.gd; tested with hop names containing /, #, and emoji. py_compile clean.