Migrate US staging run files to the version 2 contract #1135

Problem

The US fiscal-refresh build still writes schema version 1 staging run files, while UK builds use the stricter, country-neutral version 2 format introduced by #895. Both implementations write the same conceptual run bundle, but Microcosm maintains separate writer classes, fixtures, lifecycle behavior, and dashboard parsing paths.

PR #1099 separates hosted event delivery through LocalTelemetryEmitter from staging run-file persistence. It intentionally leaves the existing US file format unchanged. The next step should migrate US builds to version 2 and remove the version 1 writer.

Version 2 should be the single current staging run-file contract because it provides:

• named and validated document schemas;
• country, operation, pipeline, candidate, release, and run-kind identity;
• explicit lifecycle and delivery state;
• ordered events and consistent bundle validation;
• reviewed aggregate-only artifacts with digests and size limits;
• safeguards against uploading row-level data, credentials, archives, or dataset files;
• optional authenticated remote read-back.

Required changes

• Use the version 2 writer for every US fiscal-refresh build, including exact-count builds covered by Emit staging telemetry for US exact-K builds #933.
• Replace the two writer classes with one country-neutral StagingRunBundleWriter implementation based on the current version 2 code.
• Stop producing schema version 1 staging files.
• Remove the version 1 writer, producer fixtures, and producer-only tests after the US path migrates.
• Keep LocalTelemetryEmitter functionally independent. Disabling staging run files with --no-staging must not disable hosted event delivery.
• Populate the version 2 country, operation, pipeline, candidate, release, run-kind, sampling, and delivery fields for US builds.
• Convert US lifecycle statuses and events to the version 2 contract.
• Audit every US diagnostic currently attached to the staging run. Version 1 accepts arbitrary JSON files; version 2 permits only reviewed aggregate data and rejects record arrays, sensitive keys, dataset files, and oversized files. Summarize or omit artifacts that do not satisfy the version 2 content policy.
• Verify that Calibration Diagnostics reads new US version 2 runs before deploying the producer change. Retaining read-only support for historical version 1 runs is a separate compatibility decision and does not require retaining a version 1 writer.

Acceptance criteria

• US fiscal-refresh and exact-count builds instantiate the single shared staging run-bundle writer.
• No production Microcosm path emits schema version 1 staging files.
• Every US staging document and attached artifact passes version 2 schema and content validation.
• US run identity and delivery fields are complete and consistent across the manifest, progress document, event stream, calibration progress, and run index.
• A staging file write, validation, or upload failure cannot suppress hosted events from LocalTelemetryEmitter.
• --no-staging suppresses only staging run files and their uploads.
• Calibration Diagnostics tests cover a US version 2 run before the Microcosm producer change is deployed.
• Tests cover successful, failed, local-only, remote-delivery-failure, and exact-count US runs.
• The obsolete version 1 writer and its producer fixtures are removed.

Context

• Add versioned UK staging telemetry and non-release smoke verification #895 introduced the version 2 staging run-file contract for UK builds while explicitly retaining version 1 for US builds.
• Emit staging telemetry for US exact-K builds #933 covers enabling staging output for US exact-count builds.
• Add always-on build telemetry delivery #1099 separates the always-on hosted event emitter from staging run-file persistence.