Raise the uk extra floors to policyengine-uk 2.91

[MaxGhenis]

Pre-publication prerequisite for the first CGT-family UK release.

policyengine.py's certification gate (docs/engineering/skills/data-certification.md there) pairs a data release with a model version only when the release was built with that version or declares it in compatible_model_packages. The signed rehearsal run built against the previous lock's policyengine-uk 2.89.0, which would force the weaker publisher-claim basis when certifying against 2.91. Raising the floors so the release-candidate build records built_with policyengine-uk 2.91.0:

• microcosm-build, microcosm-data, microcosm-frame uk extras: >=2.88 → >=2.91.
• Lock: policyengine-uk 2.89.0 → 2.91.0, policyengine-core 3.26.11 → 3.31.0 (2.91 floors core at 3.30.1).
• cgt_source_stages.json policy_parameters dependency string moves in lockstep; coverage manifest regenerated.

The 2.91 line carries the retention-elasticity semantics and CGT response hardening (policyengine-uk#1803/#1806/#1823) — the same machinery the downstream reform scoring uses, so built-with and scored-with converge on one version. The CGT stage's parameter reads (Personal Allowance, AEA at 1 June 2023) are value-identical across 2.89→2.91.

Focused suites (CGT manifest, country spec, spec-only packages, release-input coverage) and repo ruff pass locally; the coverage manifest --check regenerates exactly.

🤖 Generated with Claude Code

[MaxGhenis]

Closing unmerged: CI caught a real cross-country coupling. The certified US release pins policyengine-core ==3.26.11 and its live-load consumer guard correctly refuses anything else, while policyengine-uk 2.91 floors core at >=3.30.1 — one workspace lock cannot hold both until the next US release re-certifies on newer core. Verified resolution that needs no repo change: run the UK release-candidate build under uv run --with policyengine-uk==2.91.0 — the overlay resolves pe-uk 2.91.0 with core 3.31.0 for the build process only, the lock stays untouched for every other consumer, and the release manifest records built_with 2.91.0 for the policyengine.py certification gate. Parameter values the CGT stage reads are identical across 2.89/2.91 (PA £12,570, AEA £6,000 at 1 June 2023; verified under the overlay). The floors move for real when the US lane re-certifies core.