[RNDSTROPPY-92]: pick/weighted, ENV(...) api, steps

[Cianidos]

Description of Changes

ENV() — typed environment variable accessor

Replaces raw __ENV access with ENV(name, default, description). Supports aliases (array of names), typed defaults (string/number), and descriptions. Metadata is captured by probe for the new --envs section.

const WAREHOUSES = ENV(["SCALE_FACTOR", "WAREHOUSES"], 1, "Number of warehouses");
const DRIVER_URL = ENV("DRIVER_URL", "postgres://postgres:postgres@localhost:5432", "Database connection URL");

NewPicker(seed) — weighted random selection

Provides picker.pick(items) for uniform and picker.pickWeighted(items, weights) for weighted selection. Used in new tpcc-pick.ts workload for TPC-C transaction mix (44/43/4/4/4):

const picker = NewPicker(0);
export default function (): void {
  const workload = picker.pickWeighted(
    [new_order, payments, order_status, delivery, stock_level],
    [44,        43,       4,            4,        4]) as () => void;
  workload();
}

--steps / --no-steps — step filtering

Select or exclude steps at run time without editing the script:

stroppy run tpcc --steps create_schema,load_data
stroppy run tpcc --no-steps drop_schema

Step names are validated against probed steps; unknown names produce an error.

Probe section flags

probe now supports --config, --options, --sql, --steps, --envs to show individual sections. Example stroppy probe tpcc --envs:

# Environment Variables:
  STROPPY_STEPS=""  # comma-separated list of steps to run (allowlist), same as --steps
  STROPPY_NO_STEPS=""  # comma-separated list of steps to skip (blocklist), same as --no-steps
  SQL_FILE="" (default: ./tpcc.sql)  # Path to SQL file (automatically set if .sql file provided as argument)
  DURATION="" (default: 1h)  # Test duration
  VUS_SCALE="" (default: 1)  # VU scaling factor
  POOL_SIZE="" (default: 100)  # Connection pool size
  SCALE_FACTOR | WAREHOUSES="" (default: 1)  # Number of warehouses
  DRIVER_URL="" (default: postgres://postgres:postgres@localhost:5432)  # Database connection URL

Other changes

• All workloads migrated from __ENV to ENV()
• drop_schema split into its own step in tpcb/tpcc
• tpcb/tpcds: simplified k6 options, use default export
• New tpcc-pick.ts workload (picker-based transaction mix)
• New run-tpcc-pick-test Makefile target

Motivation and Context

Scripts needed a structured way to declare environment variables with defaults and documentation. Weighted random selection was needed for realistic TPC-C transaction mix. Step filtering allows partial re-runs (e.g. skip schema creation during development).

How Has This Been Tested?

make run-tpcc-pick-test, make run-tpcb-test, make run-simple-test

Type of Changes

• New feature
• Refactoring

Checklist

• I have read the CONTRIBUTING.md
• I have checked build and tests
• I have updated documentation if needed