Fix #313: prove observer capability seam

[flyingrobots]

Summary

• replace Observer source-text seam assertions with a snapshot-backed behavior witness
• add a hostile live graph backing so the test fails if Observer bypasses the state-reader capability
• keep import/cast policy checks parser-backed through TypeScript AST traversal

Validation

• npm exec vitest run test/unit/scripts/observer-capability-seam.test.ts
• npm run typecheck:test
• npm run lint -- test/unit/scripts/observer-capability-seam.test.ts
• git diff --check
• WARP_QUICK_PUSH=1 git push -u origin fix-313-observer-capability-behavior

Closes #313

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 054ec700-c8ab-4258-b9c3-65861e5ddf32

📥 Commits

Reviewing files that changed from the base of the PR and between ab2e006 and df06d30.

📒 Files selected for processing (1)

• test/unit/scripts/observer-capability-seam.test.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix-313-observer-capability-behavior

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[github-actions]

Release Preflight

• package version: 18.0.0
• prerelease: false
• npm dist-tag on release: latest
• npm pack dry-run: passed
• jsr publish dry-run: passed

If you tag this commit as v18.0.0, release workflow will publish.

[flyingrobots]

@codex Code Lawyer Activity Summary

Issue	Severity	Source	File	Outcome
No actionable self-discovered issue found in this PR diff.	None	Self	PR diff against origin/main	No code changes made.

Validation:

Check	Result
GraphQL review-thread audit	0 unresolved, non-outdated review threads
GraphQL review audit	0 Changes Requested reviews
git diff --check origin/main...origin/fix-313-observer-capability-behavior	Pass
Added hard-ban token scan	Hard-ban diff scan found no actionable additions.