Makefile contract

The Makefile targets are part of the public tooling surface. Changes require justification in docs/maintainer/compatibility_breaks.md.

Stable targets (treated as contracts):

• install, bootstrap
• fmt, lint, test, quality, security, docs, api, hygiene, sbom
• all, clean, clean-soft
• release

Reserved/UX-parity targets:

• build: intentionally fails with guidance to use make release.
• citation: intentionally fails until citation metadata is published.

Any new target added for CI or user workflows must be documented here before release.