fix: forkchoice checkpoint-sync init — defer justified/finalized, add initing/ready status (closes #608)

[zclawz]

Summary

Closes #608

Changes

pkgs/node/src/forkchoice.zig

• Added ForkChoiceStatus enum (initing | ready)
• Added status: ForkChoiceStatus field to ForkChoice
• Genesis init (slot == 0): starts as ready (existing behaviour unchanged)
• Checkpoint-sync / DB-restore init (slot > 0): starts as initing — latest_justified and latest_finalized still hold the anchor checkpoint for internal forkchoice computation, but the node does not participate in validator duties until the first real justified update arrives via onBlock
• In onBlockUnlocked: when fcStore.update() advances latest_justified past the anchor slot, status transitions to ready and an [info] log is emitted
• Added pub fn isReady() method for callers
• Info log on init for both genesis and checkpoint-sync paths

pkgs/node/src/chain.zig

• Added .fc_initing variant to SyncStatus
• getSyncStatus() returns .fc_initing first when !forkChoice.isReady()

pkgs/node/src/validator_client.zig

• maybeDoProposal and mayBeDoAttestation now handle .fc_initing — log and return null (skip duty)

pkgs/node/src/node.zig

• Added .fc_initing to the exhaustive switch in the peer-status handler

Test Results

zig build test — 120/132 passed (12 pre-existing failures: IoUring PermissionDenied in sandbox + tree_visualizer — identical to main without changes)