feat(loop): record what the content is, as an open field (split Step 1)

[wu908]

Step 1 of the taxonomy split, deliberately invisible: the digest now also asks the model to name what this content is — 作品展示, 教程, 踩坑复盘, 清单, 观点, 前后对比 — and the name is stored on the deliverable and carried onto the project at pickup. Nothing reads it yet, so the interface does not change and the whole step can be reverted by ignoring two columns.

Why this shape rather than a fourth enum value

"What is this content" is open by nature — the creator should never pick from a list, and the model should be able to name any shape it sees. The three values that remain keep doing the job that genuinely needs a comparable finite key: routing, which judgment fields are required, which signals are watched, and (content_genome) which learned rules apply. Widening the enum instead would mean rebuilding six tables carrying the CHECK constraint.

One deliberate tolerance

The open field's length cap is 40 while the prompt asks for 4–12 characters: capping it tightly in the contract would make a slightly-too-long name fail validation and discard the entire AI draft — the silent-degradation shape behind bug #85.

Verified against the real model, with the creator's own example

material	named form	behaviour key
「第30天，新叶终于变深了」	30天养护记录	record
「春天画了六张水彩，想听听你们的意见」	水彩作品展示	share

The second is the case the creator raised — "what if I am just sharing a piece I made" — and the name now says 作品展示 instead of forcing it into 分享.

Tests

• the form is stored and mirrored in the judgment;
• the fallback path invents nothing (null);
• pickup carries it onto the project;
• one test pins the split's invariant: a project routes identically with and without a form.

Reverting the pickup carry fails that test. Full backend suite: 507 passed. Migration 057 also adds the column to the 043 rebuild DDL constant — the trap that has bitten twice.