Summary

Compute the GLOAS fork-choice head (getHeadGloas) from the incremental
indexedWeightStore instead of the full-scan weightStore. The index is
maintained on every GLOAS attestation (see #21698) but was not previously read
for head selection. Because that maintenance is GLOAS-gated, the index is cold
at the pre-GLOAS→GLOAS transition; getHeadGloas seeds it from latestMessages
on first use so it is not missing votes cast before activation.

Scoped to GLOAS only — pre-GLOAS fork choice is unchanged.

Changes

• getHeadGloas scores via headWeightStore(cs) (the index), fetching the
  justified checkpoint state before acquiring f.mu (a nil state degrades to
  zero attestation weight, as before — same pattern as pre-GLOAS getHead).
• indexedWeightStore is now structural: it stores only
  (validator, slot, payloadPresent) per target root and reads balance +
  active/slashed status fresh from the per-query justified checkpoint state
  in GetAttestationScore, instead of caching balances that go stale when the
  checkpoint advances. IndexVote no longer does a per-vote getCheckpointState
  lookup.
• RemoveVote is now allocation-free (in-place compaction). It runs per
  attestation on the GLOAS vote path, so the previous fresh-slice-per-call was a
  CPU/GC hotspot under the fork-choice lock on high-validator-count networks.
• onNewFinalized drops indexed votes for finalized-away roots.
• On the first getHeadGloas, headWeightStore seeds the index once from the
  full latestMessages snapshot (seedFromLatestMessages, clear-then-rebuild).
  Index maintenance only begins at GLOAS activation, so without the seed the head
  would undercount attestation weight at the transition until validators
  re-attest.
• Removed dead index API: the unused GetIndexedWeightStore accessor,
  GetWeightStore (orphaned once getHeadGloas switched to headWeightStore;
  NewWeightStore stays for isHeadWeak), Invalidate (cleared votes but left
  seeded set, so any future caller would silently undercount weights), and the
  write-only version counter.

Testing

• TestIndexedWeightStoreMatchesFullScan: starts from a cold index, has
  headWeightStore seed it, then asserts it returns the same GetWeight /
  GetAttestationScore as the trusted full-scan weightStore for every node in
  the filtered tree (non-vacuous) — exercising the real seed path.
• TestSeedFromLatestMessagesIsIdempotent: the seed mirrors latestMessages
  once and is a no-op thereafter (no double-counting).
• TestRemoveVoteCompactsInPlace: removal keeps the other voters and drops the
  root once empty.
• TestPruneFinalizedDropsFinalizedAndUnknownRoots: votes targeting roots at or
  below the finalized slot, or missing from the fork graph, are dropped; votes
  for live roots above it survive.
• TestOnNewFinalizedPrunesIndexedVotes: the onNewFinalized wiring empties the
  index once finalization passes every indexed root. Both pruning tests were
  mutation-checked: they fail against a no-op pruneFinalized.
• Existing fork-choice tests and cl/phase1/forkchoice: don't maintain GLOAS indexed weight store pre-GLOAS #21698's GLOAS/pre-GLOAS maintenance tests pass;
  go test ./cl/phase1/forkchoice/... and scoped golangci-lint clean.

Follow-up

The indexed store and its transition seed are superseded by the delta-propagation
rework tracked in #21704 (the canonical proto-array model both Prysm and
Lighthouse ship).