feat(language): lower hgraph IR bodies

[hhenson]

Summary

• lower typed callable and test bodies into independently owned hgraph-IR arenas
• preserve stable bindings, exact and nominal operations, substitutions, state, lifecycle, activation, traversal, assignments, and test plans
• prevent block-tail expressions from also appearing as executable statements
• extend the deterministic source-ranged dump, architecture documentation, roadmap, and focused coverage

Validation

• clang-format on changed C++ files
• git diff --check
• focused hgraph-IR suite: 262 assertions in 14 test cases
• complete language CTest suite: 29/29 passed
• clean native configure, rebuild, and CTest suite: 1711/1711 passed
• stable-ABI wheel built with Python 3.12 and complete non-WIP compatibility suite run from a fresh Python 3.14 environment: 3223 passed, 10 skipped

Stacked on #691.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-05T15:46:10.750937Z	4d288af	PR opened
🔒 Security Review	✅ Completed 2026-09-05T15:44:32.671682Z	4d288af	PR opened

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[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 4d288afe01

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
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• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud