perf(layer-loading): establish baseline and instrumentation [1/5]

[Anthony Cintron (prbatero)]

Summary

Part 1 of 5 in the project layer-loading performance stack. Establish a reproducible baseline and opt-in observability before changing runtime behavior.

Changes

• Add request-scoped logical data-layer timing and operation counters behind HASTE_PERF.
• Add deterministic 5×2, 20×5, and 50×5 synthetic fixtures.
• Add local replay, HTTP latency, and Playwright browser benchmark tools.
• Add the Compose performance overlay and feature specification.

Testing

• Phase 0 baseline reproduces 33 / 243 / 603 logical calls.
• Browser benchmark syntax check passed.
• Black, isort, flake8, and detect-secrets passed.

Stack

• This PR: 1/5, based on main.
• Next branch: prbatero/feat/perf-data-layer-core.

Review Notes

This PR intentionally adds measurement and documentation before the optimization. Runtime behavior is unchanged unless HASTE_PERF=true.