Make parallel apply compatible with RUN_STANDALONE

[SirTyson]

Description

This change makes RUN_STANDALONE compatible with parallel ledger apply. There are a few motivations for this. First, many tests enable RUN_STANDALONE, so we silently disable parallel apply in many unit tests. This change increases test coverage (and we'll need to do it eventually anyway if we make parallel apply the unconditional default). 2nd, this allows us to use parallel apply in apply load tests, which significantly reduces variance of tests like max-sac-tps test.

This touches a few tests I'm not very familiar with (and was AI assisted), but it looks reasonable to me and CI passes, but I'd like for someone more familiar with these unit tests to take a look.

Checklist

• Reviewed the contributing document
• Rebased on top of master (no merge commits)
• Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• Compiles
• Ran all tests
• If change impacts performance, include supporting evidence per the performance document

[Copilot]

Pull request overview

This PR makes RUN_STANDALONE compatible with parallel/background ledger apply so that more unit tests (and apply-load runs) can exercise the parallel apply path.

Changes:

• Allow RUN_STANDALONE to use PARALLEL_LEDGER_APPLY by removing the standalone restriction and updating manual-close/test flows to wait for background apply completion.
• Prevent virtual time from advancing while background-thread actions are queued (to avoid time jumps that can delay processing apply results).
• Adjust several tests/configs to be robust under background apply (timeouts, draining apply before shutdown, disabling parallel apply in tests that mutate headers).

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
src/util/Timer.cpp	Avoid virtual-time fast-forward when background-thread actions are pending.
src/overlay/test/TCPPeerTests.cpp	Increase timeouts to account for extra main-thread callbacks under background apply.
src/main/Config.cpp	Stop auto-disabling parallel apply in standalone; disable only for in-memory SQLite.
src/main/CommandLine.cpp	Force parallel apply in apply-load max-sac-tps mode.
src/main/ApplicationImpl.cpp	In standalone manual-close, crank until background apply finishes and LCL advances.
src/ledger/test/LedgerCloseMetaStreamTests.cpp	Drain background apply before shutdown for consistent debug txset/LCL reads.
src/ledger/LedgerManagerImpl.cpp	Improve test-only error for txset mismatch on background apply thread.
src/invariant/test/ConservationOfLumensTests.cpp	Disable parallel apply in tests that directly mutate the ledger header.
src/history/test/HistoryTests.cpp	Wait for background apply to catch up after externalization in catchup simulation.
src/herder/test/TransactionQueueTests.cpp	Wait for background apply to finish before asserting queue state.
src/herder/test/HerderTests.cpp	Wait for background apply to finish after externalization helper.
docs/apply-load-max-sac-tps.cfg	Enable parallel apply for the max-sac-tps apply-load config.

[marta-lokhova]

Thanks, this is a helpful change! Looks good overall, I just had a couple of suggestions and a question.