Cached artifact store

[azimov]

This PR introduces a content-addressable caching system for runCmAnalyses() and restructures study population creation into two phases to maximize artifact reusability when adding new outcomes.

Key Changes

Content-Addressable Caching — Artifact filenames are now derived from SHA-256 hashes of all parameters that determine their content (including databaseId). This means:

• Changing upstream settings naturally produces new filenames (no stale cache risk)
• Introduction of databaseId parameter which is used in the checksums to prevent cross database issues - when using Strategus this will be based on its own hashing mechanism further reducing collision risk
• Unchanged settings reuse existing files automatically
• No more interactive "delete old files?" prompt — the system is append-only (issue CM v6: Silently delete old files when analysis specification changes? #192)
• Two-Phase Study Population — createStudyPopulation() is split

createStudyPopulation changes

1. Base population (outcome-independent): risk window creation, censoring, minDaysAtRisk filtering. Shared across all outcomes with the same time-at-risk settings.
2. Study population (per-outcome): prior outcome removal + outcome event counting. Cheap derivation from the cached base population.

This means adding a new outcome to an existing analysis only requires the lightweight per-outcome step — all expensive shared computation (data loading, base population, PS fitting, matching/stratification) is reused from cache.

Artifact stores

Pluggable ArtifactStore interface — New R6 abstract class with LocalArtifactStore default implementation. Enables future custom storage backends (S3, shared filesystem, rdbms blob storage). Such an extension would allow for the multi-node execution of tasks and re-use of intermediate artifacts.

[codecov]

Codecov Report

❌ Patch coverage is 88.03612% with 53 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.06%. Comparing base (4f7dcea) to head (9a37a87).
⚠️ Report is 4 commits behind head on develop.

Files with missing lines	Patch %	Lines
R/RunAnalyses.R	88.85%	37 Missing ⚠️
R/ArtifactStore.R	74.28%	9 Missing ⚠️
R/StudyPopulation.R	90.78%	7 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #197      +/-   ##
===========================================
- Coverage    94.25%   94.06%   -0.20%     
===========================================
  Files           22       23       +1     
  Lines         6531     6685     +154     
===========================================
+ Hits          6156     6288     +132     
- Misses         375      397      +22     

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