feat(locate): classify duplicates semantically

[orian]

Follow-up to #245.

Summary

• keep the global repeated-definition search, including full override = true declarations
• load manifest compositions concurrently and compare object-scoped resolved schemas with the semantic diff engine
• classify results as identical duplicates, genuinely different variants, or invalid same-stack collisions
• retain source definitions and model membership for every resolved variant
• cover tables, materialized views, views, dictionaries, raw objects, named collections, abstract definitions, and no-op overrides
• print parallel-loading progress only when stderr is a terminal

Verification

• go test ./...
• go test -race ./internal/... ./cmd/...
• go vet ./...
• go build ./...
• gofmt -s -l .
• end-to-end CLI test for different variants, identical mutually-exclusive definitions hidden by override, and same-stack collisions