feat(version): add optional fixed versioning mode

[spydon]

Description

Adds an optional fixed versioning mode (lockstep) to melos version, matching the "fixed" release mode of Nx and Lerna in the JavaScript ecosystem.

melos:
  command:
    version:
      mode: fixed  # defaults to: independent

When mode: fixed is enabled, every versioning run bumps all packages in the workspace to the same new version:

• The new version is based on the highest current version in the workspace, incremented by the most significant change found in any package (a breaking change anywhere causes a major bump for all packages, a feature a minor bump, otherwise patch).
• Packages with changes keep their regular conventional-commit changelog entries; packages without changes of their own get a changelog entry noting the bump keeps the workspace in lockstep, and are listed in a dedicated section of the workspace changelog.
• A manually specified version (melos version <package> <version>) is applied to all packages. Relative manual bumps (patch/minor/major/build) are applied to the highest current version in the workspace, regardless of which package was named. Conflicting manual versions for different packages, or a manual version below a current package version, fail with a FixedVersioningException.
• Packages already at the shared version are left untouched (no pubspec rewrite, no duplicate changelog entry).
• Packages that depend on an ignored package with pending changes are skipped with a warning, mirroring independent mode's safeguard against broken releases.
• Private packages are still skipped unless --all is passed, and dependency constraints on workspace packages are updated as usual. --no-dependent-versions has no effect in fixed mode and logs a warning.
• Defaults to independent, so existing workspaces are unaffected.

Implementation notes

• The independent per-package pending updates are computed as before and then unified into one update per package sharing a single version, so all existing behavior (graduation, prereleases, changelog rendering, tagging, dependent constraint updates) is reused.
• New PackageUpdateReason.lockstep covers packages bumped only to stay in sync; it renders a dedicated changelog line and its own workspace changelog section.
• The new VersioningMode enum (independent | fixed) is exported from the public API and leaves room for future modes (e.g. grouped/linked versioning).
• Per-package git tags (foo-v1.2.3) are still created so change detection for future runs keeps working.

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 📝 Documentation update (docs/commands/version.mdx, docs/configuration/overview.mdx, docs/guides/automated-releases.mdx, melos.yaml.schema.json)
• ✅ Tests (config parsing/validation and 11 end-to-end fixed-mode versioning tests, including git-history-based bumps, manual versions, filters, and private packages)

[docs-page]

To preview the documentation for this pull request, visit the following URL:

melos.invertase.dev/~1056

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