Move the simulation executor onto the canonical SPM bundle (policyengine 6.0.0)

[MaxGhenis]

The simulation executor pins the pre-canonical bundle: policyengine==5.2.0, policyengine-core==3.30.1, policyengine-us==1.764.6, spm-calculator==0.3.1. That bundle has no measurements.spm, so runtime_spm_capability() returns None, /versions advertises no spm_capabilities entry, and every US request resolves through the legacy no-selection arm.

policyengine 6.0.0 was published to PyPI on 2026-09-15T13:37:27Z. Its bundle manifest binds the canonical SPM tuple:

• policyengine-core==3.32.5
• policyengine-uk==2.90.2
• policyengine-us==2.2.1
• spm-calculator==1.0.0
• data_releases.us -> populace-us-2024-spm-20260915
• measurements.spm -> scenario ce_trend, geography_kind county, county_vintage 2020, forecast_content_sha256 3d86d5c4c0423480e6b69b75d222ffa4a7a2639e4094df5ba2504af01be17173

Move the executor onto that bundle so the deployed worker advertises contract_version: canonical-spm-v1 with those bundle defaults.

Two things block a pin-only change:

1. policyengine-us 2.2.1 requires spm-calculator>=1.0.0,<=1.0.0.post1, which is unsatisfiable against the executor's spm-calculator==0.3.1. The automatic updater (.github/scripts/update-policyengine-package.sh) does not touch that pin, so its run for 6.0.0 failed at uv lock and opened no PR (run 34976880733).
2. BUNDLE_CONSTRAINT_FILES in src/modal/app.py selects a PIP_CONSTRAINT file by exact bundle version and raises for an unregistered one, so 6.0.0 cannot build an image until it has a reviewed calculator selection of its own.

Merging to main deploys beta and then production (simulation-deploy.yml), so the PR for this work stays a draft until a maintainer decides to release.