perf: defer tracker initialization and slim import path (1/4)

[davidberenstein1957]

Summary

Part 1/4 of the tracker performance stack (replaces #1246).

cc @inimaz — split per your review feedback into smaller, reviewable PRs.

Defers expensive work until it is actually needed so tracker construction and import codecarbon stay fast:

• Lazy-load static reference data in DataSource (CSV/JSON on first access, not at import)
• Defer hardware probing, geo validation, and emissions engine until first use
• Lazy-import output handlers and geography modules
• Skip the 1 Hz power monitor when output_methods=[]
• Avoid redundant measurement on stop() when a sample was just taken (_measure_power_and_energy_if_stale)
• Lazy-import cpuinfo, arrow, and direct output_methods imports

Benchmarks (measured locally, offline Mac ARM, 2026-06-21)

Fresh Python subprocess per sample; baseline = master.

Metric	master	After this PR	Δ
Tracker __init__ p50	190 ms	1.9 ms	~99% faster (~100×)
Cold lifecycle p50 (init→start→stop)	1,685 ms	1,767 ms	~same — hardware still probed on first start
Warm lifecycle best (same process)	1,565 ms	1,561 ms	~same — cache lands in #1252
CLI monitor subprocess p50	850 ms	786 ms	~8% faster (full CLI win in #1254)

What this PR delivers: near-instant tracker construction. The big cold start() win comes in #1252.

Stack

1. This PR — lazy initialization & import slimming
2. perf: cache hardware detection and optimize warm-path reuse (2/4) #1252 — hardware detection cache & warm-path reuse
3. perf: defer API run creation until first emission upload (3/4) #1253 — lazy API run creation
4. perf: speed up CLI monitor startup and fix wrapped commands (4/4) #1254 — CLI monitor startup

CI / quality

• pre-commit run on all changed files — passed (autoflake, isort, black, flake8)
• Targeted tests — 45 passed
• Full stacked suite (perf: speed up CLI monitor startup and fix wrapped commands (4/4) #1254 tip) — 535 passed locally

Test plan

• Reviewer: offline tracker with output_methods=[] still measures on start/stop

Replaces

Split from #1246.

[davidberenstein1957]

@inimaz — #1246 has been split into this 4-PR stack for easier review. Benchmarks in the description are measured locally (Mac ARM, 2026-06-21); pre-commit and tests pass on each branch. Please start with #1251.

[davidberenstein1957]

Test fix pushed: test_cpu_load_detection now calls _ensure_hardware_ready() (required after deferred hardware init in this PR) and mocks PowerMetrics unavailable for portable CI.

Verified locally: 504 passed, pre-commit clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.39%. Comparing base (150cac8) to head (62d2bcd).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1251      +/-   ##
==========================================
+ Coverage   89.17%   89.39%   +0.21%     
==========================================
  Files          47       47              
  Lines        4510     4565      +55     
==========================================
+ Hits         4022     4081      +59     
+ Misses        488      484       -4     

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[inimaz]

Nice thanks for the split @davidberenstein1957 . I added a comment

[davidberenstein1957]

@inimaz — addressed your review comments on input.py: restored pd.DataFrame return types on get_cloud_emissions_data and get_cpu_power_data using the TYPE_CHECKING import pattern (commit 62d2bcd).

Verified locally on all 4 PR branches:

• perf: defer tracker initialization and slim import path (1/4) #1251: 504 passed, pre-commit clean
• perf: cache hardware detection and optimize warm-path reuse (2/4) #1252: 530 passed
• perf: defer API run creation until first emission upload (3/4) #1253: 531 passed
• perf: speed up CLI monitor startup and fix wrapped commands (4/4) #1254: 535 passed

[inimaz]

LGTM thanks @davidberenstein1957 !