Adding CI for PR's -- uv lockfile verification, ruff lint, unit test suite

[jgoldberg-nvidia]

Summary

Adds a PR-level CI workflow (.github/workflows/pr.yaml) that runs on every pull request against main, motivated by the recent uv sync --extra cuda13 break in e65a857 that only surfaced after merge.

• lockfile-check job runs uv lock --locked, which re-resolves the full manifest across every Python version in requires-python and
  every extra (dev, cuda12, cuda13). Catches dependency-resolution regressions (e.g., pinning cuml-cu13==26.4.* while requires-python
  still permits Python 3.10) before merge, instead of waiting for the post-merge GPU job.
• install-and-test matrix runs unit tests on Python 3.11 and 3.12, catching code that works on the current dev target but regresses on the
  declared floor.
• lint job runs ruff format --check and ruff check on src/.
• pr-builder gate marks the PR as failing if any of the above fail.

Also switches dev tooling from black/isort/flake8 to ruff (single tool, faster) and applies ruff format to src/. Adds a
tests/test_core.py suite (38 tests) covering returns, CVaR data + parameters, portfolio math, efficient frontier, and the backtester, using the
pydantic ReturnsComputeSettings / ScenarioGenerationSettings models.

One test (test_no_fit) is skipped pending a fix to generate_cvar_data — the no_fit path passes a DataFrame to CvarData.R, which
pydantic validates as ndarray. Latent bug in main; skip preserves the test for when it's fixed.

Test plan

• lockfile-check passes on this PR (proves uv lock --locked resolves cleanly for all extras and Python versions)
• install-and-test passes on Python 3.11 and 3.12 matrix legs
• lint passes (ruff format --check src/ and ruff check src/ both clean)
• main.yml post-merge notebook run still succeeds against the merged code
• Verify locally: introduce a bad pin (e.g., cuml-cu13==99.0.*) in a throwaway commit and confirm lockfile-check fails

[phuo-nv]

@jgoldberg-nvidia we need to resolve the uv.lock conflict here. Can simply update this one. Thanks!

[Copilot]

Pull request overview

Adds PR-level CI checks to catch lockfile, linting/formatting, and unit-test regressions before merge, and migrates dev tooling to Ruff with an initial unit test suite.

Changes:

• Introduces a new PR CI workflow to verify uv.lock, run Ruff format/lint, and execute pytest across a Python version matrix.
• Switches dev tooling from black/isort/flake8 to Ruff and updates code formatting accordingly.
• Adds a broad unit test suite for core returns/CVaR/portfolio/backtest functionality.

Reviewed changes

Copilot reviewed 14 out of 15 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
.github/workflows/pr.yaml	New PR workflow for lockfile verification, linting/formatting, and tests.
.github/workflows/main.yml	Adds a lint+tests gate before running notebooks post-merge.
pyproject.toml	Replaces black/isort/flake8 with ruff + pytest in dev extra; adds Ruff config.
.pre-commit-config.yaml	Adds pre-commit hooks for Ruff lint/format plus whitespace fixers.
uv.lock	Updates lock to reflect new dev tooling (ruff/pytest) and removed black/isort/flake8.
tests/test_core.py	New unit test module covering returns, CVaR data/params, optimization, frontier, backtesting.
src/utils.py	Reformats code (notably long constants/printing and a few wrapped expressions) for Ruff.
src/backtest.py	Minor docstring edit (introduces a punctuation typo).
src/base_optimizer.py	Minor formatting change.
src/cvar_optimizer.py	Minor formatting/import consolidation.
src/cvar_utils.py	Minor formatting fixes.
src/mean_variance_optimizer.py	Minor formatting fixes.
src/rebalance.py	Minor formatting fixes (including assertions).
src/portfolio.py	Minor formatting fixes (including an assertion).
src/settings.py	Removes trailing whitespace.

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