feat(op-acceptor): add --shard-index/--shard-total for parallel gateless sharding

[smartcontracts]

Summary

• Adds --shard-index and --shard-total flags to op-acceptor for round-robin test package sharding in gateless mode
• After package discovery, packages are sorted alphabetically (deterministic), then filtered by index % total == shardIndex
• New test packages are automatically picked up by shards; removed packages disappear — no manual gate curation needed

Motivation

The Optimism monorepo's memory-all CI job runs ~40 acceptance test packages in a single worker, which is the critical path bottleneck. With this change, CI can parallelize into N shards:

# .circleci matrix
matrix:
  parameters:
    shard: [0, 1, 2, 3, 4, 5, 6, 7]

op-acceptor \
  --testdir ./op-acceptance-tests/... \
  --shard-index $SHARD --shard-total 8 \
  --exclude-gates flake-shake \
  --allow-skips --timeout 120m \
  --orchestrator sysgo

This eliminates the need for manually maintained ci-shard-* gates in acceptance-tests.yaml and orphan detection scripts.

Changes

File	Change
flags/flags.go	New --shard-index (default -1) and --shard-total (default 0) flags with env vars
config.go	Parse + validate: both must be set together, index in [0, total), gateless-only
nat.go	Thread shard config into registry
registry/registry.go	Sort packages, apply i % total == index filter in loadGatelessValidators()
registry/registry_test.go	7 new tests covering basic sharding, coverage, determinism, no-overlap, edge cases

Test plan

• TestShardFiltering_Basic — 8 packages / 4 shards = 2 each
• TestShardFiltering_Coverage — union of all shards == full package set
• TestShardFiltering_Deterministic — same shard twice = same result
• TestShardFiltering_NoOverlap — no package in multiple shards
• TestShardFiltering_MoreShardsThanPackages — empty shards handled
• TestShardFiltering_SinglePackage — 1 package / 2 shards
• TestShardFiltering_Disabled — default values = no sharding
• All 18 registry tests pass (11 existing + 7 new)
• go vet clean on all changed packages

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 54.54545% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.79%. Comparing base (6b0f7b2) to head (3b46771).

Files with missing lines	Patch %	Lines
op-acceptor/config.go	0.00%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #566      +/-   ##
==========================================
- Coverage   57.82%   57.79%   -0.03%     
==========================================
  Files          96       96              
  Lines       14477    14508      +31     
==========================================
+ Hits         8371     8385      +14     
- Misses       5591     5608      +17     
  Partials      515      515              

Flag	Coverage Δ
op-acceptor	58.97% <54.54%> (-0.07%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
op-acceptor/flags/flags.go	81.81% <ø> (ø)
op-acceptor/nat.go	33.21% <100.00%> (+0.15%)	⬆️
op-acceptor/registry/registry.go	82.17% <100.00%> (+0.68%)	⬆️
op-acceptor/config.go	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes

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[scharissis]

We have a very similar PR as WIP here: #553

And the discussion which lead to it here:
https://oplabs-pbc.slack.com/archives/C09E7EJGN5P/p1770986805525969

Did you happen to see/consider any of this beforehand? I am wondering if we can streamline our efforts.