introspect: widen supported engine set (S3, integration engines, VersionedCollapsing, Graphite, ...)

Symptom

The engine switch in internal/loader/hcl/introspect.go:529-689 covers the MergeTree family (+ Replicated variants), Distributed, Log, Join, Null, Memory, Merge, Buffer, Kafka and TimeSeries. Everything else falls through to "unsupported engine" and aborts introspection (or requires -allow-raw).

Missing: S3, URL, File, MySQL, PostgreSQL, MongoDB, RabbitMQ, NATS, EmbeddedRocksDB, Dictionary-engine tables, VersionedCollapsingMergeTree, GraphiteMergeTree, and their Replicated variants.

Impact

Any database containing one of these engines cannot be introspected strictly; with -allow-raw the objects degrade to opaque raw{} blocks (text-diffed, DROP+CREATE on change).

Fix direction

Add typed engine specs incrementally, prioritized by what real deployments use (VersionedCollapsingMergeTree and the integration engines are the likeliest). Each addition per project rule needs introspect + dump + sqlgen + diff coverage.

Found in the 2026-07-02 deep-dive audit (docs/plans/2026-07-02-deep-dive-improvement-areas.md, B2).

Start with internal/loader/hcl/introspect.go:529-689 and the existing engine switch. Choose one prioritized missing engine, then trace the project rule for adding typed engine specs and locate the corresponding introspect, dump, sqlgen, and diff coverage. Done means that engine no longer falls through to unsupported engine and all four paths are covered.