fix: root format implies unified versioning

[Slokh]

When using format = "root", all non-ignored workspace packages are now implicitly treated as one fixed group. This means:

• All packages share the highest bump level from any changelog entry
• The root CHANGELOG.md gets a single version heading (no duplicates)
• No need to manually configure [[fixed]] groups when using root format

This makes format = "root" behave as expected for monorepos shipping a single product. The config simplifies from:

dependent_bump = "patch"

[[fixed]]
members = ["common", "wallet", "request"]

ignore = ["sign", "test"]

[changelog]
format = "root"

To just:

ignore = ["sign", "test"]

[changelog]
format = "root"

Backward-compatible: explicit [[fixed]] groups still work, per-crate format is unaffected, and single-package workspaces are unchanged.

Includes the heading-merge fix from #75 plus a new golden test (root-fixed-group) and updated root-changelog golden.

[github-actions]

⚠️ Changelog not found.

A changelog entry is required before merging. We've generated a suggested changelog based on your changes:

Preview

---
changelogs: minor
---

Added support for unified versioning in root changelog format by implicitly treating all workspace packages as a fixed group, added workspace version inheritance support for Rust (`version.workspace = true`), and fixed deduplication of changelog entries when multiple packages share the same version in root format.

Add changelog to commit this to your branch.