Epic: pub.dev depth — dependency-graph wiring, dependent constraint rewriting, Flutter build numbers

[goosewobbler]

From the July 2026 direction review (§1.2). Dart is releasekit's least-contested ecosystem (nobody else combines versioning + release PR + GitHub Releases + OIDC publishing for Dart), but support has a correctness hole vs what melos users expect.

Scope

• Wire pub into the dependency graph — packages/version/src/core/workspaceGraph.ts:8 says "pub: not yet wired (no edges) — deferred". Until fixed, prerequisite derivation and publish ordering silently ignore Dart packages.
• Dependent constraint rewriting: pub has no workspace:* indirection at publish time — when package A bumps, dependents' pubspec.yaml constraints must be rewritten, preserving style (caret vs exact pin), across dependencies and dev_dependencies (melos's core value; currently a correctness hole for multi-package Dart repos).
• Flutter +N build-number retention: preserve/increment build metadata across bumps (1.2.3+11 → 1.2.4+12).
• Graduation cascade parity: match melos version --graduate semantics (strip prerelease → stable including dependents) within releasekit's channel model.

Later (separate issue when wanted)

A pre-publish pana score gate (the Very Good Workflows pattern) — no non-Dart tool will ever have this.