fix(staleness): guard priceTimestamp+timeout overflow; treat as stale

[thedavidmeister]

Summary

• LibFtsoCurrentPriceUsd.ftsoCurrentPriceUsd: wrap the priceTimestamp + timeout addition in unchecked with an overflow guard — if the sum wraps below priceTimestamp, treat it as stale instead of panicking with Panic(0x11).
• testRunStaleTimestampOverflow: fuzz test covering the new path, bounding priceTimestamp so priceTimestamp + timeout always overflows and asserting StalePrice (not a panic).

Test plan

• testRunStaleTimestampOverflow passes (was previously impossible to reach the StalePrice path because checked arithmetic would panic first).
• Existing testRunStale and testRunStaleBoundaryNotStale still pass.
• CI green.

Closes #105

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: 64a2cd14-21ec-40d5-82e9-c3d00aaaac97

📥 Commits

Reviewing files that changed from the base of the PR and between cb20d03 and 29ff19a.

⛔ Files ignored due to path filters (1)

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

📒 Files selected for processing (2)

• src/lib/price/LibFtsoCurrentPriceUsd.sol
• test/src/lib/op/LibOpFtsoCurrentPriceUsd.t.sol

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch issue-105-staleness-overflow

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[thedavidmeister]

🤖 ai:vetter
Reviewed 29ff19a: ready — Correct overflow guard: unchecked block with deadline<priceTimestamp detection treats overflow as st
cost 415 — unchecked overflow guard in staleness check plus fuzz test

[thedavidmeister]

Closing in favor of #125 (kept). #125 fixes the same overflow via subtraction (block.timestamp > ts && block.timestamp - ts > timeout) across BOTH LibFtsoCurrentPriceUsd.sol (#105) and LibFtsoV2LTS.sol (#102), treating a large timeout or a future/large priceTimestamp as "never stale" (staleness is only about being too old). This PR reverts StalePrice on overflow — which mislabels a large timeout (a valid "never stale" config per #102) as stale. Keeping the single #125.