Migrate field-create onto the field-add lifecycle driver

[HynLcc]

What

Third and widest member of field-add-lifecycle.ts — 4 cases: 5 simple fields, 5 formula fields, mixed-19, and single-select with 1k options.

field-create seeds an empty (base-fields-only) table, adds N fields in one measured trace step with per-field engine routing, optionally polls until the formula columns finish their computed backfill (a DB aggregate over all rows), verifies the created fields and single-select options, then restores the seed by deleting the added (non-base) fields.

Unlike the prior two members its prepare is a single measured phase (prepareFieldCreate in execute; a synthetic zero-duration seedBuild/seedRestore in seed mode), so prepareFixture owns that measurement and parks its name+duration on the (mutable, in-place) fixture; buildResult rebuilds the prepare measurement from the live object after seedReady/backfill mutate it. The driver is unchanged — a third runner, with the most divergent shape yet, slots in with zero driver edits. Cleanup re-resolves created fields by "not a base field", the same invariant assertSeedReady already enforces.

diff-artifacts.mjs masks

All proven volatile by an unmodified-main baseline A vs B:

• field-create generated field ids: createdFields[].id, ready.computedFields[].id, ready.dependencyFields[].id, the flat details.fieldIds list, and the compiled details.verifiedFields[].expression (embeds A/B/C ids), plus details.ready.dbTableName.
• details.prepare.seedHash / seedTableName — the same content-addressed seed-cache key the cache.seedHash rule masks for other runners (stable run-to-run, moves only on refactor; the seed config that also feeds it is frozen because cases/** is untouched).

Field names and computedFields[].expectedKind keep the semantic identity visible.

Validation (local, all 4 cases, v1+v2)

• Baseline A vs B clean after the noise masks.
• pnpm check green.
• G1 baseline vs candidate clean for all 8 artifacts.
• Negative tests: phase / per-field routing.routeMatched / computed-backfill checks.mismatches / ready.totalRows / createdFields[].name / single-select verifiedOptions[].name drift all caught; only the seedHash + field-id keys ignored (over-mask check: createdFields[].name still caught).
• Per-field routing asserts v2 actualXTeableV2=true, feature createField; formula backfill verified 10000/10000, 0 nulls / 0 mismatches.
• Trace ref counts unchanged (6/6/20/2).
• Shared driver untouched → no sibling regression needed.

Tracker: 22/35 runner kinds, 29/55 cases. This closes the field-add family campaign (conditional-lookup #59, field-duplicate #60, field-create here).

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