Preserve deliberate Unicode replacement characters in DSP export #306

🤖 Generated by the Agentic Engineer

Part of #283.

The catalog accepts valid UTF-8 containing a deliberate Unicode replacement character, but offline DSP export rejects that scalar as if decoding had lost data.

For publishers, operators and product consumers, the expected result is consistent validation and honest public metadata. The hypothesis is that enforcing the existing boundary at its real caller prevents this failure without adding steps to the valid workflow.

Acceptance criteria:

• Accept legitimate literal and escaped replacement characters without accepting malformed UTF-8 or unpaired surrogate escapes. Test real catalog-handler responses through the offline exporter, including emoji controls.
• Prove the regression fails on current main and passes with the smallest root-cause repair.
• Keep affected documentation consistent and deliver through reviewed, passing CI and an actual merge.

Success signal: the focused regression and valid-path controls pass within this delivery round. Size: small to medium.