## What changed
Export `ElasticBudgetConfig`, named `ElasticSection` groups, and validated `ElasticSectionLimits` for optional evidence. Each group can specify a minimum token reservation and a per-entry text cap.
Validate limits during construction and deserialization: both values must be positive, and the entry cap must fit within the minimum reservation and the existing text-part delivery allowance. Reject unknown configuration fields.
This adds configuration types only; elastic allocation is not activated.
## Testing
Add a serialization round-trip test and deserialization checks for zero limits, entry caps above the minimum reservation, and entry caps above the delivery allowance.
GitOrigin-RevId: c869018cfc0d96377b1adbfbdd98e1d88bd1b32a