release: v0.8.0

[vyncint]

Cuts 0.8.0. Notes are the [0.8.0] section of CHANGELOG.md, which
extract-changelog.sh turns into the GitHub Release body.

One finding, filed from the repository that consumes this one (#97, fixed in
#98): --track withheld its placement suggestion from exactly the plans that
could still act on it. A picture that could not be drawn where it sat was told
so and nothing else, while the only thing left to do about holed is move it.

Why minor and not patch

cargo-semver-checks against 0.7.0, before the bump:

Checked 196 checks: 195 pass, 1 fail, 0 warn, 58 skip

--- failure function_parameter_count_changed ---
  mossaic::plan::best_start_week now takes 7 parameters instead of 6

docs/RELEASING.md calls a changed public item breaking, and pre-1.0 that is a
minor. Binaries and the Action are unaffected — only a caller of the library
directly.

Checklist

• gh workflow run stress.yml — green (run)
• Cargo.toml and the Cargo.lock entry at 0.8.0
• [Unreleased] moved to [0.8.0] - 2026-09-06, both link definitions repointed
• extract-changelog.sh 0.8.0 returns the section; test-extract-changelog.sh passes
• step 2b's grep is clean — four action@v0.7.0 refs and two "pin a number"
  examples moved; the remaining matches are history (fixed in 0.7.0 in
  SECURITY.md, until 0.4.0 in DESIGN.md) and fixture data in the
  changelog extractor's own test
• fmt, clippy -D warnings, full test suite, cargo doc with RUSTDOCFLAGS=-D warnings

After merge: tag v0.8.0 on main, which publishes.