fix(nzb-postproc): process every PAR2 recovery set, not just the first

[thedancingdeveloper]

Summary

Ports the still-open bug fix from MrVampy/rustnzb (b1c3a45): the post-processing pipeline only ever inspected par2_files[0].

The bug (present on main)

When a single job carries more than one independently posted release — each with its own PAR2 recovery set — only the first set was verified, repaired, and deobfuscated. The remaining sets were silently ignored:

• their obfuscated volumes were left un-renamed on disk (Extract then finds "no archives"), and
• any damage in the later sets went unrepaired.

This is distinct from main's recent PAR2 work (WI-143 damage ledger, WI-144 slice verifier, WI-151 fixtures) — none of those touch recovery-set selection in pipeline.rs.

The fix

Both the zero-article-failure (skip + deobfuscate) branch and the verify+repair branch now enumerate every distinct recovery set:

• parse_par2_sets — skips .volNNN+NNN.par2 volume files (they carry recovery blocks, not an index), parses each remaining index, and dedupes by recovery_set_id so duplicate indices for the same set collapse.
• verify_repair_set — verifies one set and, if damaged, repairs it from the pre-computed verify result (no redundant second pass). Run for every set inside a single spawn_blocking; stage messages are tagged Recovery set N/M.
• detect::is_par2_volume is now pub(crate).

Port notes

Adapted to main's pipeline_ok failure signalling — the fork's commit depended on its own typed JobFailureCode enum (a separate change, ported in another PR), whose failure_code = Some(...) assignments are dropped here.

Tests

• pos_multiple_recovery_sets_each_restore_their_files — posts two recovery sets (rustnzbfixture01/02) with two obfuscated volumes and asserts both are deobfuscated. It fails on the old single-set logic (one obfuscated volume would remain). Adds a Par2Fixture::with_recovery_set_id builder.
• cargo test -p nzb-postproc → all pass (80 across the suite); cargo clippy -p nzb-postproc --all-targets, cargo fmt --check, cargo check --workspace → clean.

Original author: @MrVampy (credited via Co-Authored-By; ported by hand due to divergence in pipeline.rs).

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