feat(bam): populate @PG header with PN/VN/CL fields

[pinin4fjords]

Summary

The BAM @PG line in rustar was content-free - just @PG\tID:rustar-aligner, no PN, VN, or CL fields. Downstream provenance tools (MultiQC program-version table, dx-toolkit lineage tracking, internal QC dashboards) end up with a blank entry where STAR shows a fully populated record.

This PR expands the @PG line to match STAR's format and the SAM spec §1.3 conventions:

@PG\tID:rustar-aligner\tPN:rustar-aligner\tVN:<cargo pkg version>\tCL:<command line>

The full command line is captured via std::env::args() in main.rs before clap consumes it, then threaded into Parameters via a new #[arg(skip)] field (command_line: Option<String>) so it reaches build_sam_header_from_refs in src/io/sam.rs. Version comes from env!("CARGO_PKG_VERSION") at compile time. When command_line is None (e.g. library use, internal callers), CL falls back to "rustar-aligner" so the field is still non-empty.

Scope

This addresses only Gap 1 of #33 (the @PG content). Gap 2 (AS value divergence on ~2.4% of identical-CIGAR records) is left to a separate investigation since its root cause appears to overlap with #27 (sjdb seeding).

Test plan

• New unit test test_build_sam_header_pg_line_populated asserts the @PG line carries PN:rustar-aligner, VN:<cargo pkg version>, and CL:<expected command line>
• New unit test test_build_sam_header_pg_line_default_cl_when_unset covers the command_line = None fallback so CL is still non-empty
• All 385 existing lib tests still pass (no test asserted the prior content-free format)
• cargo build
• cargo clippy --lib -- -D warnings clean
• cargo fmt --check clean

Refs #33 (Gap 1 only)

[pinin4fjords]

Verified end-to-end on macOS/aarch64 against the rebuilt fix branch.

samtools view -H <bam> @PG line:

@PG  ID:rustar-aligner  PN:rustar-aligner  VN:0.1.0  CL:/tmp/rustar-fix-33/target/debug/rustar-aligner --genomeDir idx-rustar --readFilesIn ... --outFileNamePrefix VP.

Pre-fix the same line was just @PG\tID:rustar-aligner (no PN, no VN, no CL). After the fix all four fields are populated — PN and VN from the cargo package, CL from the captured std::env::args(). Matches STAR's format. LGTM.