fix: preserve files of all sources in multi-source backup

[Lauri-Nomme]

Problem

Backing up more than one source path in a single call silently drops the second source's files — or panics with StripPrefixError.

Repro:

rustic_core$ cargo test --test integration test_backup_multi_source_preserves_files_of_all_sources

before this change:

thread '<unnamed>' panicked at crates/core/src/archiver.rs:163:66:
called `Result::unwrap()` on an `Err` value: StripPrefixError(())

Reported upstream: rustic-rs/rustic#1905

Root cause

Archiver::archive received only backup_paths[0] as the single backup_path used for strip_prefix (crates/core/src/commands/backup.rs). The ignore-based walker emits absolute paths per source root for every entry. With multiple sources, an entry from the second source (.../src2/file.txt) cannot strip_prefix(backup_paths[0]), so:

• with --as-path set: the unwrap() panics,
• without --as-path: the second source's file blobs never get archived while its directory tree is still written — a silent, successful-looking backup missing data.

Fix

Pass the full backup_paths slice into Archiver::archive and strip each entry against the backup root it actually belongs to (falling back to the entry path unchanged if no root matches). When --as-path is used with multiple sources, keep each source as its own subtree under the target to avoid collisions between sources that contain identically-named files.

Tests

Two regression tests added in crates/core/tests/integration/backup.rs:

• test_backup_multi_source_preserves_files_of_all_sources (with --as-path)
• test_backup_multi_source_preserves_files_without_as_path (default mode)

Verified:

cargo test -p rustic_core            # 168 passed
cargo test --test integration        # 59 passed
cargo clippy -p rustic_core --all-targets   # no warnings

Notes

This changes the signature of Archiver::archive (backup_path &Path → backup_paths &[PathBuf]). It is pub, so downstream users calling it directly would need to update — but the normal entry points (repo.backup, repo.archive) are unchanged.

[Lauri-Nomme]

Retracting per maintainer feedback: as-path+multi-source is already rejected in the rustic CLI, and the no-as-path path needs a cleaner verification. Will pursue root-cause analysis and reopen/replace with a validated fix.