Module Federation: name the source in trace output #2796

Problem

--trace-file and --trace-dependency do not say when a file or a
dependency comes from Module Federation.

Current behavior

For a file exposed through exposes:

fallow dead-code --trace-file src/Button.tsx --format json --quiet

Returns is_entry_point: true with no source field.

For a remote alias registered through remotes:

fallow dead-code --trace-dependency checkout --format json --quiet

Returns is_used: true with no marker that the name is a Module
Federation remote rather than an npm package.

Expected behavior

Both trace commands name Module Federation as the source when that is why
the file or the dependency is credited.

Acceptance criteria

• --trace-file on an exposed file adds a source field naming Module
  Federation and the config that exposes it.
• --trace-dependency on a remote alias adds a marker that the name is a
  Module Federation remote, not a package.
• Both fields are additive; docs/backwards-compatibility.md records
  them; npm run generate:contracts:check passes.
• A trace on a file or a dependency with no Module Federation involvement
  is unchanged.
• CHANGELOG entry.

Relevant files

• crates/core/src/plugins/module_federation.rs
• crates/cli/src trace command handling