fix(sflr): revert on zero rate from sFLR contract

[thedavidmeister]

Summary

• Adds ZeroSFLRRate error to src/err/ErrFtso.sol
• Guards LibSceptreStakedFlare.getSFLRPerFLR18() against a zero return from the sFLR proxy — reverts with ZeroSFLRRate instead of forwarding a zero exchange rate to oracle consumers
• Adds testGetSFLRPerFLR18ZeroReverts() using vm.mockCall to verify the guard fires
• Regenerated FlareFtsoWords.pointers.sol with updated bytecode hash

Closes #80

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: 1c6a90cd-6299-4b18-85f5-5af29bd2e750

📥 Commits

Reviewing files that changed from the base of the PR and between cb20d03 and 32eb4c7.

⛔ Files ignored due to path filters (1)

• src/generated/FlareFtsoWords.pointers.sol is excluded by !**/generated/**

📒 Files selected for processing (3)

• src/err/ErrFtso.sol
• src/lib/sflr/LibSceptreStakedFlare.sol
• test/src/lib/sflr/LibSceptreStakedFlare.t.sol

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch issue-80-sflr-zero-check

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