Format generated now/now_on_write values to the field's declared type

[chrisstrahl]

Bug

Any field that pairs a date type with a generated timestamp makes create() and update() fail their own validation:

updated:
  type: date
  generated: now_on_write

generateValue() (and the inline now_on_write stamping in update()) always emit new Date().toISOString() — a full ISO datetime — regardless of the field's declared type. The operation stamps 2026-07-23T06:09:41.123Z into a date field, then validation rejects the value it just generated with invalid_date, and the whole write fails with validation_failed.

validate() passes on the same files because the on-disk values are proper YYYY-MM-DD dates; the invalid value is introduced by the write path itself. So collections in this shape validate clean but can never be written through the API — every update fails even when the requested field change is a no-op.

Repro (v0.2 profile, default_validation: error): declare the field above, create a file with a valid date value, then call collection.update({path, fields: {title: "x"}}) → validation_failed / invalid_date on updated.

Fix

• generateValue() formats now / now_on_write to the declared field type: date → YYYY-MM-DD, time → HH:MM:SS, anything else → full ISO datetime (all UTC, consistent with the existing {today: true} lifecycle provider and the validator's UTC-based Date coercion).
• The inline now_on_write stamping in update() now goes through the same helper instead of a hardcoded toISOString().
• The v0.2→v0.3 type migration maps now / now_on_write on date-typed fields to the {today: true} provider instead of {now: true}, so migrated collections don't inherit the same failure.

Tests

New test/generated-date-fields.test.ts covers create, update, and migration for date/datetime/time-typed generated fields (all three failed before the fix). Full suite: 2005 passed; the one failing test (v03-conformance lifecycle conflict diagnostic) fails identically on a clean checkout of main and is unrelated.

Found in production use: a knowledge-base collection where every type declares updated: {type: date, generated: now_on_write} — mdbase update was unusable across the entire collection.

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