fix(protocol): decouple preconf metadata existence from anchorBlockNumber

[dantaik]

What

Fixes the Anchor.getPreconfMetadata existence sentinel, which collides with a valid anchorBlockNumber == 0.

Closes #21762.

Problem

getPreconfMetadata used anchorBlockNumber != 0 as the "metadata exists" sentinel (Anchor.sol:208), but BlockParams.anchorBlockNumber is documented as "0 to skip" (Anchor.sol:36). A block legitimately recorded with anchorBlockNumber == 0 was therefore indistinguishable from one that was never recorded, causing getPreconfMetadata to revert and blocking PreconfSlasherL2 from validating faults for that block (or its predecessor, which also reads the next block's metadata). _validateBlock enforces no anchorBlockNumber != 0 guard, so nothing on-chain prevents the collision. Currently latent (the driver passes real L1 block numbers), but the fix removes the dependence on off-chain driver behavior for a slashing-safety invariant.

Change (minimal)

• Add an explicit bool stored marker to PreconfMetadata and use it as the existence sentinel in getPreconfMetadata.
• Set stored: true in _storePreconfMetadata (called unconditionally for every block).
• This preserves the documented anchorBlockNumber == 0 "skip" semantic, rather than removing it (which a naive require(anchorBlockNumber != 0) would do).

Storage-layout safety

stored packs into the existing first slot alongside the three uint48 fields (18 + 1 = 19 bytes ≤ 32), so rawTxListHash / parentRawTxListHash keep their sub-slots and the contract-level layout is unchanged: _preconfMetadata stays at slot 258 and __gap at slot 259 (Anchor_Layout.sol is unaffected). Please regenerate via pnpm layout in CI to confirm. Note: any PreconfMetadata entries written before this upgrade would read stored == false; this is acceptable for the rolling/recent preconf window but worth confirming there are no live pre-upgrade entries that must remain readable.

Tests

• test_getPreconfMetadata_recordsBlockAnchoredAtZero — a block anchored with anchorBlockNumber == 0 is now retrievable (the regression).
• test_getPreconfMetadata_RevertWhen_BlockNotRecorded — an unrecorded block still reverts with InvalidBlockNumber.

Note: forge could not be run in the authoring environment (network policy blocked the toolchain install), so compilation/tests rely on CI here. The change is small and the diff is included for review.

Reviewers

Requesting @ggonzalez94 and @davidtaikocha.

https://claude.ai/code/session_0187MFZTJHwwQ6zUhRus39we

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