Give freeform an upper witness without restoring a length cap

[atomchung]

User outcome

A simple lookup stays short. A research-heavy decision can carry several sourced increments when each one changes the call, instead of being implicitly compressed toward a corpus in which every positive exemplar was under 400 characters.

Current behavior / evidence

• Move limits to claim, write, and action boundaries #828 moved missing-input limits from whole-turn refusal to claim/write/action boundaries; its frozen owner blind test preferred the candidate in 3 of 4 scenes.
• feat(challenge): delete the obligation whitelist, keep the seven facts that earn their place (closes #830) #831–Put one canonical exemplar into each surface's generation path (closes #834) #835 then removed obligation recitals and put canonical exemplars onto the generation path. The frozen rerun materially shortened all four target scenes.
• [design·expression] The exemplar corpus is a character cap the contract twice refuses to have #836 found the remaining contradiction: the contract rejects a character cap, but the binding positive corpus demonstrated only the short consequence and almost no multi-source research depth.

Change

• retain the existing compact freeform positives as lower witnesses;
• add one synthetic freeform_research_depth positive with eight distinct increments, explicit source/as-of provenance, an engine-owned portfolio consequence, a contextual research judgment, and observable falsifiers;
• make that scene the one exemplar visible in references/freeform-answers.md;
• state beside it that this is an upper witness, not a minimum/default answer size or permission to narrate irrelevant research.

Scope / non-goals

This is deliberately one surface. It changes no engine, schema, persisted state, transaction lifecycle, execution truth, privacy boundary, or other conversational route. consider, no_book, and weekly_read remain unchanged until owner-live evidence shows they need the same treatment.

Acceptance

• existing compact freeform scenes remain in the corpus;
• freeform stays inside the corpus's 3–5 positive-exemplar range;
• the new scene passes E-5 through E-8;
• the reference/corpus drift gate proves the generation-path copy is canonical;
• product and QA/eval CI pass;
• follow-up owner-live evidence compares one simple lookup and one research-heavy decision before this pattern expands to another surface.

Privacy / rollout / recovery

All issuers, ratings sources, prices, dates, holdings, and deal facts are synthetic. The change is reversible by restoring the previous freeform fence and removing one fixture scene; no migration or recovery path is involved.

Refs #836. This PR does not close it: the issue remains open until owner-live evidence decides whether any other surface needs an upper witness.