feat: add SCI report output method

[davidberenstein1957]

Adds an SCI (Software Carbon Intensity, ISO/IEC 21031:2024) output method, following the same pattern as the existing BoAmps exporter.

What it adds

codecarbon/output_methods/sci.py, a single module holding the two declaration dataclasses, the pure mapping function, and the handler:

• FunctionalUnit / EmbodiedDeclaration — the R and M terms, which only the user can supply.
• map_emissions_to_sci(data, ...) — pure, no I/O. I is derived as emissions * 1000 / energy_consumed rather than recomputed, so the report agrees with the CSV by construction and inherits the cloud-region, PUE and country/region handling already applied upstream.
• SCIOutput(BaseOutput) — out() writes sci_report_<run_id>.json, task_out() writes sci_report_tasks_<run_id>.json with one entry per task, live_out() is a documented no-op because the report is final rather than incremental.
• OutputMethod.SCI registered in _init_output_methods, reading an optional sci_context_file from the usual config hierarchy.

Two honesty rules, both tested:

• No R declared → the report is still written, with sci: null and a status field explaining why, instead of dividing by zero or assuming R = 1.
• No M declared → M_gCO2e: 0 with M_source: "not declared", so a reader can see the report covers only the operational half.

User-facing surface

from codecarbon import EmissionsTracker
from codecarbon.output_methods.sci import FunctionalUnit, SCIOutput

sci = SCIOutput(functional_unit=FunctionalUnit(name="inference request", count=10_000))
tracker = EmissionsTracker(output_handlers=[sci])

sci.set_functional_unit_count(n) for the common case where the count is only known at the end; SCIOutput.from_file("sci_context.json") for the declarative path; output_methods = csv,sci plus sci_context_file in .codecarbon.config.

Docs: a new SCI section in docs/reference/output.md (including a sample report), a row in docs/how-to/examples.md, and a runnable examples/sci_output.py. No new dependencies.

Verification

uv run pytest tests/test_sci_output.py -q — 14 tests covering the formula, provenance assembly, the undeclared-R and undeclared-M branches, zero-energy, the handler and its task path, and context-file loading (valid, missing, malformed). black --check and ruff check were run scoped to the touched files; the remaining ruff hits are the same Optional/List/dict() style the surrounding modules already use, so the new code matches its neighbours.

Deliberately left out

• Any automatic embodied-carbon estimation: no EmbodiedProfile tracker parameter, no bundled hardware table, no cloud per-instance figures. CodeCarbon has no defensible manufacturing data, and an uncited M in a compliance report is worse than a blank one. M is user-declared or explicitly undeclared.
• Any change to what emissions means. Existing consumers are untouched.
• Multiple simultaneous functional units, and schema validation on write (there is no stable machine-readable SCI schema to validate against yet).

Closes #1353

🤖 Generated with Claude Code

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Review follow-up: embodied carbon in task reports

task_out() passed the same declared M to every task, so a run with 5 tasks reported the device's full embodied carbon 5 times, and each per-task sci included 100% of it — a wrong number in a document framed as ISO/IEC 21031 output.

M is now apportioned by each task's share of the run duration. Chosen over omitting M from task reports because the split is exhaustive: R is already the run-level count, so with an apportioned M the per-task figures sum back to the run-level report instead of quietly dropping half the accounting. M_source records the share applied, e.g. "vendor LCA; apportioned by duration (25.0% of the run)".

Tested by test_task_out_apportions_embodied_by_duration (unequal durations: asserts the 10/30 split of a declared 40 gCO2e and that the shares sum back to 40), plus the undeclared-M and zero-duration branches.

Also documented in docs/reference/output.md that the configuration-only path (output_methods = csv,sci) can never produce a non-null sci unless functionalUnit.count is hardcoded in the context file, since set_functional_unit_count() needs a reference to a handler the tracker owns.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.60%. Comparing base (065d0e6) to head (e06fdc3).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1361      +/-   ##
==========================================
+ Coverage   91.39%   91.60%   +0.20%     
==========================================
  Files          49       50       +1     
  Lines        5056     5156     +100     
==========================================
+ Hits         4621     4723     +102     
+ Misses        435      433       -2     

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