refactor: extract microflow anchor selection helpers

[hjotha]

Part of #332.

Closes #324.

Summary

Extracts duplicated microflow builder anchor selection logic into two helpers. This is a no-behavior-change refactor intended to make follow-up anchor/handler fixes smaller and easier to review.

Root cause

Several builder paths repeated the same precedence checks inline for pending anchors, branch anchors, and statement anchors.

Refactor

• Add branchDestinationAnchor.
• Add pendingFlowAnchors.
• Route existing IF and graph construction call sites through the helpers.

Behavior

No behavior change intended. Behavior-changing fixes remain in separate branches.

Tests

No new behavior tests added because this only extracts existing logic. Existing tests cover the affected paths.

Validation

• make build
• make test
• make lint-go
• make test-integration

Agentic Code Testing

• No user-facing syntax or behavior change.
• Existing test suite covers the refactored builder paths.

Test plan

• Full test suite remains green after refactor.

[github-actions]

AI Code Review

Critical Issues

None found.

Moderate Issues

None found.

Minor Issues

None found.

What Looks Good

• Refactor quality: The duplicated anchor selection logic has been cleanly extracted into two well-named helper functions (branchDestinationAnchor and pendingFlowAnchors). The refactor is applied consistently across all identified call sites in the microflow builder.
• Scope adherence: This is a pure refactor with no behavior changes, as stated and validated. The PR description clearly limits the scope to extracting duplicated logic.
• Test coverage: Existing tests cover the refactored paths (confirmed by validation checklist: make test, make test-integration pass). No new behavior tests were needed per the refactor nature.
• Code placement: Helpers are added to cmd_microflows_builder_flows.go alongside the existing applyUserAnchors function, which is logical and maintains cohesion.
• Validation: Author verified build, test, lint, and integration tests pass. Agentic checks confirm no user-facing syntax/behavior change and existing test coverage.
• Pattern consistency: All instances of the original inline anchor precedence checks have been replaced with the helpers, eliminating duplication without altering control flow.

Recommendation

Approve. The refactor improves code maintainability with zero behavior risk, follows the project's atomic commit principles, and has been thoroughly validated. No changes are needed.

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