bug: root cargo workspace releaser rewrites member crate versions, overriding per-path releases

## Environment details

- OS: nixos (linux)
- Node.js version: 24.14.0
- npm version: 11.9.0
- `release-please` version: 17.5.2

## Summary

When using manifest mode with a Cargo workspace, configuring root path `.` as `release-type: rust` rewrites all workspace member crate versions to the root version, even when member crates have separate release paths/config or no releasable changes.

This causes inconsistent output (for example member `Cargo.toml` version rewritten while manifest entry stays unchanged).

## release-please version

Observed in action logs: `release-please version: 17.1.3` (`googleapis/release-please-action@v4`).

## Repro config

```json
{
"plugins": [{ "type": "cargo-workspace", "merge": false }],
"packages": {
".": {
"release-type": "rust",
"exclude-paths": ["crates", "editors"]
},
"crates/panache-parser": {
"release-type": "rust"
},
"editors/code": {
"release-type": "node",
"package-name": "panache-code"
}
}
}
```

Workspace members:

- `crates/panache-parser`
- `crates/panache-wasm`

## Steps to reproduce

1. Have last released versions around:
- root `.` = `2.33.0`
- `crates/panache-parser` = `0.2.0`
- `crates/panache-wasm` = `0.1.1`
2. Push a commit that triggers only root release (`.`), with no `crates/panache-parser` changes.
3. Run manifest release PR.

## Actual behavior

Logs show root Rust strategy doing workspace-wide update:

- `found workspace with 2 members, upgrading all`
- `updating 2 submodules`
- versions map sets `panache`, `panache-parser`, and `panache-wasm` to the root next version (e.g. `2.33.1`)

PR diff includes:

- `crates/panache-parser/Cargo.toml` bumped to root version (`0.2.0 -> 2.33.1`)

while manifest proposal can still keep parser unchanged (`"crates/panache-parser": "0.2.0"`), leading to inconsistency.

## Expected behavior

In manifest mode with per-path packages:

- Root `.` release should not force member crate package versions to root version.
- Member crate versions should be controlled by their own path candidate/config, or remain unchanged if no releasable changes exist for that path.
- Cargo workspace dependency updates should respect per-package release decisions and keep manifest/Cargo.toml consistent.

## Likely root cause

In `src/strategies/rust.ts`, workspace handling unconditionally sets member versions to root `newVersion` (`versionsMap.set(member.package.name, version)`), which conflicts with manifest per-path semantics in monorepos.