Release 0.8.0

[orgoca]

Merge #73 first. This branch sits on top of it. Until #73 lands, this diff also shows #73's commit; after that it is only the version bump, changelog heading and roadmap.

What 0.8.0 contains

PR	Change
#69	retention_by_segment: retention, decision agreement across PD cutoff ranges, and band edges in range, per segment. sweep_whitebox(segments=), train_whitebox(sample_weight=), and a refusal for uncalibrated artifacts (closes #12)
#70	GitHub Discussions, linked from the new-issue page and CONTRIBUTING
#72	The Where it fits page; the docs read beyond credit risk without hiding its origin
#73	Why this exists opens with the credit origin and states the choice in general terms

Why a minor release

retention_by_segment, segments and sample_weight are new public API. git diff --stat v0.7.1..HEAD over runtime/, export/ and docs/ARTIFACT_SPEC.md is empty.

This PR

• __version__ → 0.8.0, and CITATION.cff version → 0.8.0.
• A [0.8.0] changelog heading with a summary. The three PRs' sections are merged into one Added and one Changed; previously they read Changed / Added / Changed.
• The README roadmap's "Recently shipped" becomes a list and gains 0.7 (PD and reason codes from the exports), which was never added, and 0.8.

ruff, black, pytest and mkdocs build --strict pass locally.

After merge

Wait for the matrix on the merge commit to go green, then tag that commit:

git checkout main
git pull
git show HEAD:src/compileml/__init__.py | grep __version__   # must print 0.8.0
git tag v0.8.0
git push origin v0.8.0

Then publish the release from v0.8.0 and approve the pypi gate.