Add configuration preflight mode

[Toti330]

Summary

Adds PRINT_CONFIG=1, a no-side-effect configuration preflight mode.

Why this is useful

Performance and database investigations often start with uncertainty about the effective environment. This prints the exact resolved seed range, thread counts, batch/channel settings, database target, and logging mode without starting a workload or touching PostgreSQL.

Implementation

• Prints resolved non-secret runtime configuration and exits successfully before connection, checkpoint reads, thread spawn, or file changes.
• Always redacts PG_PASS.
• Validates WORKER_THREADS, WRITER_THREADS, and COMMIT_COUNT are non-zero before work begins.
• Adds usage documentation in docs/config-preflight.md.

Independence

This PR is based directly on main and is independently mergeable. It has no dependency on the timing, diagnostics, tuning, checkpoint, or progress PRs.

Verification

• cargo test --all-targets passed.
• PRINT_CONFIG=1 smoke run printed the configured values and config.database.password=redacted, with no database connection attempt.

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Generated by GPT-5.6 Terra & Sol. Ideas from the author; implementation by AI.

[SuperB3333]

Nice debugging feature. I'll implement some readability changes and commit this later

[SuperB3333]

Implemented in 0176b8b