Sweep declared manifest outputs against written columns before they become load-bearing

Follow-up from the PR #690 review (workstream E of #665; see also #145).

The class

The licensed-data acceptance on #690 caught a declared manifest output (state_pension) that was an SPI concept, not a persisted frame column — dormant for as long as outputs were purely declarative, refused at full rung the moment country_stage_plan compiled outputs into StagePlan.produces. As the review put it: the other manifests may carry the same latent class, dormant until they too become load-bearing. Each discovery via a build costs a credentialed full-rung run; a static sweep costs one pass.

Scope

Validate declared outputs against what the runtimes actually write, statically, for every source manifest not yet load-bearing:

• us/source_stages.json (39 stages) against the us_plan stage produces declarations and runtime constants — note the US plan has stages with no manifest entry, so the mapping needs care rather than a naive zip.
• be/source_stages.json against its implementations.
• uk/cgt_source_stages.json (the bespoke CGT declaration from #676) against cgt_imputation's written columns — also a candidate for absorption into the canonical uk/source_stages.json, which would make its outputs load-bearing the same way.
• The frozen uk/hmrc_income_source_stages.json is a historical evidence anchor with one known latent entry (the state_pension concept) — document, don't edit; it retires at the FRS 2024-25 refresh (#687).

PR #690 already pins the two migrated UK stages' outputs to named runtime written-column constants (TestDeclaredOutputsAreWrittenColumns); this issue extends the discipline to the rest.

Acceptance

A conformance test (or one per country package) that fails when a declared output names a column no runtime constant or plan declaration backs; every existing latent entry either fixed or documented with an owner.

Start with the existing TestDeclaredOutputsAreWrittenColumns coverage from PR #690, then inspect us/source_stages.json, be/source_stages.json, uk/cgt_source_stages.json, the us_plan declarations, and cgt_imputation's written-column constants. Account for US stages without manifest entries and keep uk/hmrc_income_source_stages.json unchanged. Done means conformance coverage catches unsupported outputs and every latent entry is fixed or documented with an owner.