move BadMinStackLength to ErrFlow.sol

[thedavidmeister]

Summary

Every other error reverted from Flow.sol is declared in a dedicated error file (ErrFlow.sol or rain.factory's ICloneableV2.sol). BadMinStackLength was the only outlier — declared inline in the implementation file.

Moved to src/error/ErrFlow.sol so consumers (interfaces, ABI bindings, integrators) can get the selector without importing the concrete Flow contract and its OZ Multicall / ReentrancyGuard / interpreter dependencies.

Closes #414.

Test plan

• forge build clean

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: 780cb0da-68bb-4da4-ab99-72e6eaf6874e

📥 Commits

Reviewing files that changed from the base of the PR and between ceaea3c and 3fd7294.

📒 Files selected for processing (2)

• src/concrete/Flow.sol
• src/error/ErrFlow.sol

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch 2026-05-06-issue-414-badminstacklength-relocate

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