Ship native Hummingbird read-only server

[DandyLyons]

Summary

• register generic Hummingbird 2 collection, item, and logical-path handlers from EndpointPlan
• add the md-utils-server executable with .md-utils/server.yaml loading and recursive immutable project import
• add stable JSON error envelopes, collision-safe 409 responses, documentation, Linux CI, and in-process route coverage

Verification

• swift test (1,123 tests passed)
• swift package generate-documentation --target MarkdownUtilitiesServer --warnings-as-errors
• swift run md-utils-server --help
• focused Hummingbird/project-loader tests (5 passed)
• local Docker execution unavailable because the Docker daemon was not running; Native Server Linux CI provides Swift 6.2 verification

Closes #77