Split live backfill checkpoint helpers

[yvette-carlisle]

Summary:

• Split live baseline backfill checkpoint/source-identity helpers into backfill_checkpoint.rs.
• Kept backfill.rs focused on env knobs, note_input, SQL state lookups, duplicate detection, async add_note/resume orchestration, and report assembly.
• Added focused tests for source/corpus hashing, checkpoint reset/load/write roundtrip, and checkpoint entry validation reject paths.

Validation:

• cargo make fmt-rust
• cargo nextest run -p elf-eval --all-targets --all-features backfill_checkpoint (4 passed)
• cargo make lint-vstyle
• cargo make fmt-rust-check
• git diff --check
• cargo make check-rust
• cargo make lint-rust
• cargo make test-rust (390 passed, 92 skipped)
• cargo make check (390 passed, 92 skipped)

Review:

• Scout selected live backfill checkpoint/source identity as a strong boundary.
• Skeptic passed with no blockers or material risks after adding stored source_hash validation coverage.