Build K epic: transactional country executor via strangler extraction from the US release monolith

Goal (architecture review, recommendation 5)

Replace the imperative US release monolith (tools/build_us_fiscal_refresh_release.py, ~6,600 lines) with a transactional country executor: CountrySpec compiles into a typed DAG of idempotent stages — intake → source/support construction → target materialization → selection/calibration → export → gates → release — where every stage reads and writes content-addressed artifact manifests and is independently restartable. Specs then govern the build (resolving #159's open question) instead of reviewing it.

Method: strangler, not rewrite — the monolith is the oracle

1. Identity spine first (prerequisite, = review recs 2+3, extends #326): define BuildRunIdentity (raw-input revisions/SHAs, facts+manifest SHA, country-spec fingerprint, donor hashes, selection identity-set SHA, seed + stage config, populace commit, PE-US/Core versions, canonical post-source frame hash) and the constraint-system fingerprint (CSR indptr/indices/data, target vector, stable record IDs, engine + materializer identity). Every stage keys its cache and its output manifest on these. This is design-sensitive (silently-wrong-if-wrong) — spec reviewed at Fable/Max tier before implementation.
2. Executor skeleton + toy conformance country: a minimal synthetic country runs build → gates → publish → immutable-load end-to-end in CI. This test is the platform's definition of done and every extraction must keep it green.
3. Extract stages one at a time from the monolith behind the executor interface, newest-and-cleanest first (gates and export are already near-modular; target materialization has checkpointing; the solve gets its long-missing checkpoint here). Per-stage acceptance = byte-identical outputs vs the monolith on a pinned fixture run — the monolith stays authoritative until each stage's parity is proven, then that stage is deleted from it.
4. Operational payoff is built-in: stages are naturally <30-minute processes with durable state between them — the jetsam discipline stops being a doctrine and becomes the architecture; a late-stage failure can no longer orphan a release manifest (each stage commits its manifest atomically; release marking is the final atomic step).
5. Country migration order: US (it is the monolith) → UK (proves the abstraction on the second consumer; the ladder + local stack slot in as stages) → BE/NZ (should be nearly config-only if the abstraction is real). #366's release-delta report becomes trivial on staged artifacts (diff manifests stage-by-stage).

Sequencing vs live work

Build J's re-certification ships on the CURRENT monolith (the SSI fix does not wait for a re-platform). The identity spine (step 1) starts now — it hardens the monolith immediately and the executor inherits it. Steps 2-5 are the Build K campaign proper, running parallel to launch-tier paper/site work.

Division of labor

• Design (Fable + Max review): BuildRunIdentity + constraint-fingerprint field spec; the stage-DAG contract; per-stage acceptance criteria.
• Sol (gpt-5.6-sol, reasoning ultra) builds: per-stage extractions against the parity oracle, fingerprint implementation after spec sign-off, the toy conformance country — bounded, single-repo, loud-failure work.
• Claude agents: gate/certification semantics, release operations, cross-repo integration, review of every sol PR, issue/PR conventions.
• Human-only: this epic's go/no-go and its sequencing against the launch window.

Refs: the 2026-07-09 architecture review (codex-populace-architecture-review-20260709.md), #159, #326, #366, #368/#369 (gate machinery the executor absorbs), #305 (launch META).

Start with tools/build_us_fiscal_refresh_release.py and the referenced architecture review, then read #159 and #326 for the identity-spine context. The first milestone is a reviewed BuildRunIdentity and constraint-fingerprint specification; eventual completion requires a toy conformance country in CI and byte-identical stage outputs against the monolith on a pinned fixture.