docs(sync-service): trim heavy comments and inline single-use helpers

[tcoratger]

Summary

Documentation and minor-structure cleanup of the sync orchestrator. No behavior change — process_block, the gossip handlers, replay, proof recovery, and the state machine are untouched in logic. The file drops 105 lines, almost all comment/docstring weight.

Changes

• One-line docstrings for the retryable-attestation error, the publish-callback and block-aggregates fields, and the block proof-recovery method.
• Proof-recovery comments trimmed from a ~50-line essay to one-sentence-per-line notes, keeping only the non-obvious reasoning (parent-state key resolution, the order-invariant fold, proof supersession).
• Dropped the list() wrap around the block attestations — the SSZ list is a concrete sequence, iterated natively.
• Removed banned markdown-bold Why: labels in the persistence prune comment; the genesis and finalized-root rationale is now plain prose.
• Shortened the block-processing, recovery-call, and write-back comments to the load-bearing facts (keeping the no-await write-back invariant the review flagged).
• Inlined two single-use helpers (component wiring into post-init; the block-aggregate publish loop into the gossip handler) and deleted them.

Considered and deliberately kept

The review weighed several more removals that were not applied, because they are multi-caller methods encapsulating a shared invariant (inlining would duplicate guard logic and risk drift):

• _deconstruct_block_into_store stays a method on the orchestrator — relocating a 165-line isolated function to its own file is relocation, not decoupling.
• _integrate_gossip_attestation / _integrate_gossip_aggregated_attestation — each has two callers (live + replay) and centralizes the unknown-block rejection remap.
• _transition_to — two callers plus two tests pinning the invalid-transition guard.
• has_root / head_slot — the read-only StoreView protocol implementation that backfill depends on.

Not addressed (flagged for a follow-up)

The aggregated-attestation gossip handler increments no valid/invalid metrics while the single-attestation handler does — likely an oversight, but a behavior change, so left out of this docs pass.

Testing

• just check passes.
• tests/node/sync/test_service.py passes (35 tests).

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