Add txset construction mode to apply load

[SirTyson]

Description

Adds a new mode to apply load, "txset-construction-and-apply". This mode times all leader work in a ledger, without any network work: tx set construction, validation, then application. It's the leader end-to-end variant of txset-validation-and-apply, which simulates a non-leader node with only tx set validation and application. In testing, it seems like the number of TXs in the queue has a significant impact on TX set building times, so unlike other apply load modes, this test will generate more TXs than will actually fit in the final block in order to overfill the queue.

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

Adds a leader-side txset-construction-and-apply benchmarking mode to measure transaction-set construction, validation, and application.

Changes:

• Adds queue-overfilling construction mode and timing metrics.
• Extends configuration, tests, and benchmark examples.
• Documents the new mode and metrics.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Review
src/simulation/test/LoadGeneratorTests.cpp	Adds SAC coverage; missing construction-mode coverage for CUSTOM_TOKEN and SOROSWAP (nit).
src/simulation/ApplyLoad.h	Declares construction timing helpers and state.
src/simulation/ApplyLoad.cpp	Implements construction timing, but duplicates and omits production nomination behavior and inflates global queue metrics (moderate).
src/main/Config.h	Adds the timing-mode enum value.
src/main/Config.cpp	Parses the new configuration value.
src/herder/HerderImpl.h	Declares the transaction-set build timer.
src/herder/HerderImpl.cpp	Instruments construction, but measures a mismatched span (moderate) and lacks performance evidence (nit).
docs/software/commands.md	Documents the mode as production nomination despite its synthetic implementation (moderate).
docs/metrics.md	Documents the build metric.
docs/apply-load-ledger-limits.cfg	Documents queue multiplier configuration.
docs/apply-load-for-meta.cfg	Documents queue multiplier configuration.
docs/apply-load-benchmark-token.cfg	Documents token benchmark configuration.
docs/apply-load-benchmark-sac.cfg	Documents SAC benchmark configuration.

Suppressed comments (1)

src/herder/HerderImpl.cpp:1669

• The new production-path timer adds unconditional clock reads and metric updates to every leader nomination, but the PR provides no performance evidence (the checklist item is unchecked). CONTRIBUTING.md:56-60 requires evidence of improvement and no regression for performance-impacting changes; please include measurements that quantify this instrumentation and the new benchmark mode before merging.

    auto const txSetBuildStart = std::chrono::steady_clock::now();

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