[mjanez]

A profile field with control: asset stores asset IDs. A connector only knows MRNs, so it could not fill one: the value was stored as text the sheet does not recognise.

• On write, a link written as mrn://… in an asset-control field is replaced by the ID of the asset it names. An MRN that names nothing is left out. Applies to any writer (API included); plain IDs are checked as before.
• In a run, the first pass leaves out whole any link field with an MRN that does not resolve yet (DeferLinks), so a partial value is never stored and then mistaken for a curated one. After the batch, the assets that carried MRNs are written once more, now that every target exists.
• Ceiling, marked in the code: an asset the run finds unchanged is not revisited, so a target another source adds later is linked the next time the asset changes.

Also moves the two database tests of internal/core/asset to asset_test: pgtest reaches the package through the plugin registry, and the import cycle kept the whole package's tests from compiling.

Tests: go test ./internal/core/asset/ ./internal/core/runs/ ./internal/core/contentpack/, with and without MARMOT_TEST_POSTGRES_DSN; new TestProcessEntities_LinksByMRNAnAssetCreatedLaterInTheSameRun and TestAnMRNLinkResolvesOnceItsTargetExists.