"Waystone's lane invariant is the proof that collapsing the two dates creates sentinel dates and silent data loss."
Vesta Didn't Discover This Rule — It Inherited It
The two-truth rule can sound like clever foresight, but Vesta didn't reason its way to it from a blank page. It inherited it from its parent. Waystone already carried the same split under different names: captured_local_date, the day a breadcrumb was physically taken in the field, versus the schedule-owner's local_date, the day of the trip that breadcrumb belongs to. Those are the same two questions — when it happened versus which day it's filed under — and Waystone kept them in separate fields because it had learned, in the field, that it had to.
That learning is what Waystone's lane invariant encodes: it is the proof that collapsing the two dates produces sentinel dates and silent data loss. Waystone met the failure on a real trip, hardened the rule against it, and wrote the invariant down. Vesta reads that invariant as an inheritance and starts on the far side of the bug — it gets the correct model for free, without having to re-suffer the outage that taught it.
Why Rules That Look Like Ceremony Aren't
Here's the danger: to a fresh reader, 'keep two date fields separate' can look like over-engineering. One date is simpler; why carry two? That exact reasoning is how the bug gets re-introduced — someone collapses the fields to 'simplify', ships it, and slowly rediscovers the sentinel dates and lost write-times that Waystone already paid for. The invariant exists precisely because the simplification is seductive and wrong. Reading the lineage — knowing this rule is scar tissue, not decoration — is what stops a future builder from cutting it.
The Bridge to the Rest of the Quest
This is your first real taste of what it means for Vesta to be a child engine, and the whole final track is about it. Almost every hard rule in Vesta is an inheritance from Waystone, carried forward with its field bruises intact. The two-truth rule is just the clearest example: the parent met the failure, the child gets the fix. That relationship — inherit the hard-won contract, diverge only where the domain truly differs — is the deepest thing this quest has to teach.