Deprecate region:"us"/"uk" as the national request spelling

[anth-volk]

Context

National macro requests are spelled two ways today:

• Omit region — the sim API's own convention (contract tests, integration tests, docs examples).
• region: "us" — what API v1 sends on every national US macro run (SimulationOptions.region is a required string in policyengine-api/policyengine_api/services/economy_service.py:79; the value is stored in a NOT NULL DB column on job records). v1 is the only production caller using this spelling.

The worker accepts both via the normalisation shim at simulation_runtime.py:212-218 (None/empty/"us"/"uk" → country), and as of #637 the segmented-national eligibility does too (is_plain_national_macro). Two spellings for the same semantics means every national-detection site must remember both — #637's review caught exactly this class of drift (eligibility initially treated region:"us" as regional, which would have excluded all v1 production traffic from segmentation).

Goal

One national spelling: omit region. region:"us"/"uk" becomes deprecated, then rejected.

Migration checklist

Phase 1 — v1 stops sending it (policyengine-api):

• economy_service.py: make SimulationOptions.region optional; send no region for national runs (:79, _setup_sim_options :972-1006, _setup_region :1137-1149)
• _classify_simulation_geography (:1254) currently detects national via region == country_id — needs the omitted-region case
• reform_impact table: region VARCHAR(32) NOT NULL — decide sentinel (e.g. national) or schema change
• Routes that 400 on missing region query param (economy_routes.py:52, :105-107)

Phase 2 — contract + budget window (policyengine-sim-api):

• BudgetWindowBatchRequest.region is a required str (gateway_models.py:178) — national budget windows currently must send region:"us"; make it optional with omitted == national (golden + client regen)
• Document in gateway_models.py that national == omit region (or use region_group)

Phase 3 — deprecation signal, then removal (policyengine-sim-api):

• Emit a deprecation warning + telemetry attribute in the _normalise_region_code shim (simulation_runtime.py:212-218) when region in {"us","uk"}
• Once telemetry shows zero region:"us" traffic: remove the shim, remove the region in ("", country) acceptance in segmented_national.is_plain_national_macro, and reject the spelling with a clear error

Interim state

Until Phase 1 lands, the sim API MUST keep treating region:"us" as national everywhere — it is the production national traffic. #637 aligned segmentation eligibility with the worker shim so both spellings behave identically.