Add stigma in-memory engine scaffolding

[fabricekabongo]

Summary

• add a new stigma package with cell indexing, ingest pipeline, and aggregate stores
• implement simple WAL persistence and a minimal HTTP wrapper
• cover ingest, transition, query, and WAL replay behaviors with unit tests

Testing

• go test ./...

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Codex Task

[deepsource-io]

DeepSource Code Review

We reviewed changes in ece3789...74c5fd0 on this pull request. Below is the summary for the review, and you can see the individual issues we found as inline review comments.

See full review on DeepSource ↗

PR Report Card

Overall Grade   Focus Area: Hygiene

Security   Reliability   Complexity   Hygiene

Code Review Summary

Analyzer	Status	Updated (UTC)	Details
Docker		Feb 25, 2026 10:05p.m.	Review ↗
Secrets		Feb 25, 2026 10:05p.m.	Review ↗
Test coverage		Feb 25, 2026 10:05p.m.	Review ↗
Go		Feb 25, 2026 10:05p.m.	Review ↗

Code Coverage Summary

Language	Line Coverage (New Code)	Line Coverage (Overall)
Aggregate	57.5%	67.5% [▼ down 0.5% from main]
Go	57.5%	67.5% [▼ down 0.5% from main]

➟ Additional coverage metrics may have been reported. See full coverage report ↗