fix: validate web feature entries

[nanookclaw]

Summary

Fixes #510.

This adds a validation step inside V3Parser.Parse before the existing post-processing logic runs. The validator checks the raw features object from data.json so malformed entries fail closed instead of being silently skipped or ingested with empty display data.

The checks now reject missing/empty feature maps, empty feature IDs, malformed feature values, unknown or missing kind discriminators, and kind: "feature" entries whose name is missing or blank. I left release-version handling in the existing workflow layer: parseByVersion already gates parser selection from the GitHub release tag, and the current upstream data.json does not include a top-level version field.

The post-processing switch now returns an error for bad discriminator data as a second guard, and feature processing rejects empty names before constructing the stored feature value.

Verification

• make jsonschema to generate ignored local schema code needed for tests
• GOTOOLCHAIN=go1.26.0 go test -count=1 ./pkg/data
• git diff --check

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