Report all non-fatal import skips via the warnings channel + typed warnings

[njakobsen]

The feature_update_warnings channel (added in #53) currently only carries the NetworkLink message. The importer silently drops several other non-fatal things:

• invalid individual placemarks (currently collected into @errors, which raises unless skip_invalid)
• blank / zero-geometry features (blank_feature?, geom blank)
• GDAL field-width truncations (e.g. Value … of field Shape_Area … not successfully written)

Proposal: route these non-fatal skips through the same warnings channel so a partial import reports what didn't import and why — not just NetworkLinks. This turns the warning surface into a general "here's what was skipped" report, which is what an uploader actually wants.

Related: consider storing warnings as typed/structured data ({type:, file:, payload:}) instead of pre-formatted strings. That would:

• make the reword in Reword the KML NetworkLink import warning #54 trivial (formatting moves to the display layer),
• allow categorization / per-warning handling,
• let consumers (connect_engine) format/localize messages themselves.

Larger change than #54; prioritize independently.

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