perf(ya): defer heavy imports at startup — Δ=-38ms (p=0.0026, BCa CI [12ms, 58ms])

[KorsarOfficial]

Defer tier-1 Python imports for faster bootstrap

Summary

Move heavy tier-1 imports (devtools.ya.core.report, app_config) behind
importlib.import_module() call sites and add PEP 562 __getattr__ lazy
re-exports to four modules in devtools/ya/core. This eliminates eager
import graph traversal during ya --help and reduces bootstrap overhead.

Evidence

Inner-container wall-clock timing (docker exec, n=20, 3 warmup runs):

Metric	Before	After	Delta
Median	1.025s	0.987s	-38ms (-3.7%)
Mean	1.027s	0.990s	-37ms
Std	41ms	20ms	-51% variance

Statistical validation:

• Mann-Whitney U = 312, p = 0.0026 (two-sided), significant at α = 0.05
• Effect size r = 0.78 (large)
• BCa 95% CI for improvement: [12ms, 58ms] (n_bootstrap = 10000)

Disclosure: outer Docker timing (measuring container startup + Python)
shows p = 0.091 (not significant). Container startup overhead dominates
at that granularity (~2.4s total vs 38ms signal). Inner-container timing
isolates Python execution and is the authoritative measurement.

Supplementary: full statistical analysis available in the rendered
optimization evidence report (reporting/output/07-optimization-evidence.tex).

Changes

devtools/ya/core/monitoring/__init__.py
  - Remove eager `import devtools.ya.core.report as report` at module level
  - Change report_type default to None; resolve inside MetricStore.store()
  - Add PEP 562 __getattr__ for lazy re-export of report module symbols

devtools/ya/core/stage_tracer/__init__.py
  - Defer app_config import behind call site

devtools/ya/core/stages_profiler/__init__.py
  - Defer heavy import behind call site

devtools/ya/core/yarg/handler.py
  - Add PEP 562 __getattr__ for lazy re-export of handler symbols

Net change: 4 files modified, ~40 lines changed.

Patches

• patches/15-01-task1-defer-tier1-imports.patch (apply first)
• patches/15-01-task2-pep562-lazy-reexports.patch (apply second)

Task 2 must follow task 1. Both patches together constitute this PR.

Testing

# Build
ya make devtools/ya/bin

# Unit tests
ya test -tt devtools/ya/core

See upstream/test-results.log for test execution status and environmental
constraints. Historical validation: patch applied in yatool Docker container
during Phase 15 implementation; ya make devtools/ya/bin succeeded.

CLA

I hereby agree to the terms of the CLA available at:
https://yandex.ru/legal/cla/?lang=en

[KorsarOfficial]

Evidence Report

Full statistical analysis for this optimization:
https://github.com/KorsarOfficial/yatool/releases/download/v1.0-perf-analysis/07-optimization-evidence.pdf — Section 2: Python Bootstrap (p.4)

Also available:

• findings.pdf — Top 5 bottlenecks summary
• 05-profiling.pdf — Profiling methodology

All reports: https://github.com/KorsarOfficial/yatool/releases/tag/v1.0-perf-analysis