Fix #376: Assert BitmapNeighborProvider backend invariant

[flyingrobots]

What changed

Replaced the unreachable hasNode() fallback false with an explicit IndexError invariant guard and added focused coverage for that impossible route.

Why

The old tail branch made an invalid backend state indistinguishable from a real missing node. The provider now fails closed if readiness says a backend exists but routing cannot find one.

Closes #376

Validation

• npx vitest run test/unit/domain/services/BitmapNeighborProvider.test.ts
• npm run typecheck:src -- --pretty false
• npm run typecheck:test -- --pretty false

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 03044336-6112-4926-b781-645d6fb682c0

📥 Commits

Reviewing files that changed from the base of the PR and between a21fb8b and d1238f7.

📒 Files selected for processing (2)

• src/domain/services/index/BitmapNeighborProvider.ts
• test/unit/domain/services/BitmapNeighborProvider.test.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix-376-bitmap-neighbor-provider-assertion

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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GraphQL review-thread audit	0 unresolved, non-outdated review threads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-376-bitmap-neighbor-provider-assertion	Pass
Added hard-ban token scan	Hard-ban diff scan found no actionable additions.