Document Feature and RelationMember fields in feature.proto

[brawer]

What

Comment-only change: documents every field of Feature and RelationMember in feature.proto. Prompted by reviewing doc clarity after #631 landed centroid_s2_cell_id/coverage_s2_cell_id on FeatureToIndex — that message ended up well-documented, but Feature/RelationMember had real gaps.

Gaps fixed

• tags: gave no clue it's flattened (key_id, value_id) StringPool-index pairs. That fact only existed in a private helper's rustdoc in assemble.rs, invisible from the proto itself.
• way_members vs RelationMember.id: both uint64, one message apart, look parallel — but way_members holds raw OSM node IDs while RelationMember.id holds id-encoded feature IDs (osm_id*10 + {1,2,3}). Nothing warned a reader they're differently encoded.
• RelationMember.role: no comment at all — not obviously a StringPool index rather than an enum or the literal role string.
• version/changeset/timestamp: undocumented, no units, no note that they default to 0 when OSM doesn't supply them.
• geometry_wkb: didn't say it's little-endian WKB with (longitude, latitude) coordinate order.
• No message-level note on which fields apply to which OSM element kind (way_members ways-only, relation_members relations-only).

Also gave Feature the same "internal/ephemeral, never sent over the wire or stored long-term, field numbers are free to be renumbered" note FeatureToIndex already had. Since Feature is declared first in the file, the full explanation lives there and FeatureToIndex just refers back to it, so a top-to-bottom reader gets the full story once, in the right order.

Fixed two pre-existing typos in FeatureToIndex's trailing comment (should't → shouldn't, finalm → final) while in the area.

All field semantics were checked against the actual population code (assemble_tags, assemble_relation_members, assemble_feature in assemble.rs), not guessed.

Testing

cargo build (regenerates the prost bindings from the proto) and cargo fmt --check: clean. No .rs changes, so no behavior change.

🤖 Generated with Claude Code

[brawer]

LGTM