Mid-Period Tax Rate Change with Version-Controlled Rate Tables

A jurisdiction changes a VAT/GST/sales-tax rate effective mid-period (e.g., a rate moves from 19% to 20% on the 15th). The tax administrator loads the new rate as a new **effective-dated version** of the rate rule rather than overwriting the existing one. Transactions before the effective date continue to resolve to the old rate;
transactions on/after resolve to the new rate. Both versions remain queryable for audit and amended-return scenarios. No in-place edit destroys the prior rate.

Given a jurisdiction with a VAT rate at 19% effective before a boundary date and 20% effective on/after;
when a transaction with tax_point_date before the boundary is posted;
then the 19% rate is applied;
when a transaction with tax_point_date on/after the boundary is posted;
then the 20% rate is applied;
both rate versions remain queryable;
(negative) when an admin attempts to overwrite an in-force rate version in place then 409 with error code rate_version_immutable.

tax_rate_version { id: string, jurisdiction_code: string, rate_percent: string, effective_from: date, effective_to: date, created_by: string, created_at: timestamp, source_authority_ref: string, external_id: string };
append-only;
referenced by tax_line;
(reference, product may differ).

POST /v1/tax-rate-versions { jurisdiction_code, rate_percent, effective_from, effective_to, source_authority_ref, external_id } -> 201 { id, effective_from, effective_to };
GET /v1/tax-rate-versions?jurisdiction_code=&at_date=;
emits tax_rate.version_created event;
idempotent via external_id.

SCHEDULED -> ACTIVE -> SUPERSEDED;
terminal SUPERSEDED;
guard: overlapping effective windows rejected on load with 422 rate_version_overlap;
no in-place edit of an ACTIVE version permitted.