fix(backup): resolve helper libraries from script dir when ODS_DIR lib is missing

[vaibhavsrv]

Why this matters

In ods-backup.sh, shared rsync and path utilities are sourced directly from $ODS_DIR/lib/rsync.sh and $ODS_DIR/lib/backup-paths.sh. When an operator runs ods-backup.sh against a custom target installation directory (or a minimal fixture directory lacking a checked-out lib/ directory), sourcing fails immediately with:
ods-backup.sh: line 32: /path/.../lib/rsync.sh: No such file or directory
This crashes the backup utility before arguments can be inspected or preflight disk checks can run.

This change adds a fallback: when $ODS_DIR/lib/rsync.sh is absent, the script resolves its helper libraries from $SCRIPT_DIR/lib/ (the directory where ods-backup.sh itself resides). Existing in-tree installations and backup retention policies remain unaffected.

Validation

• Baseline reproduction: Setting ODS_DIR to a directory lacking lib/ and invoking ods-backup.sh aborted with No such file or directory.
• Post-fix verification: Running ods/tests/test_backup_script_lib_fallback.py verifies that ods-backup.sh seamlessly resolves helper libraries from SCRIPT_DIR/lib and executes with exit code 0.
• Telemetry: Backup test suite passes: test_backup_script_lib_fallback.py passes cleanly (exit code 0). Wired into Linux CI workflow under Legacy Update Fallback Backup Round Trip.

Overlap check

• Issue Disk-space preflight suite never reaches the preflight: its fixture install has no lib/ #5482: Resolves the missing library fixture dependency identified when testing backup/restore preflight checks against minimal ODS directory structures.
• ods rollback exits 1 without a message and restores nothing after ods update (looks in data/backups, which ods update never creates) #5375: Discussed rollback directory discovery. This change focuses strictly on library sourcing fallback within ods-backup.sh.
• Does not alter backup tar formats, rsync filters, or retention counters.

Risk / AI disclosure

AI-assisted investigation, implementation, and test regressions. This introduces fallback library resolution when running against non-standard ODS_DIR paths. Independent human review and platform/runtime qualification remain gates. No running configuration, deployment or upstream merge changed.

Follow-up integration evidence

Composed with #5871, #5951, #5952, #5953, #6076, #6077, #6078, and #6079 at HEAD without conflicts. Production and test diffs passed together; backup and restore suites remain intact.
Backlog composition was local-only (production/test diffs, excluding workflow/Makefile wiring); it is not an upstream merge or independent human approval. Declared live-review gates remain open.