feat(merge): add command to merge two pangenome graphs

[mmolari]

Adds pangraph merge <LEFT_GRAPH> <RIGHT_GRAPH>, which combines two existing graphs into one. Covers §4.2 and §4.5 of dev/design/merge.md.

Arguments (§4.2). The five parameters that actually govern a merger — aln_args, alignment_kernel, max_self_map, extra_band_width, max_alignment_attempts — move out of PangraphBuildArgs into a GraphMergeParams struct in align/, which build and merge both flatten. The deep call chain (merge_graphs, solve_promise, reconsensus_graph, edit_consensus_and_realign, map_variations) now takes &GraphMergeParams, which also removes the align → commands::build dependency. No flags are added or removed from build; the only change to its help output is that -c/--circular now appears after -k instead of before -x.

Identifier relabeling (§3.4). Two independently built graphs always collide — Pangraph::singleton labels the first genome of every build with path, block and node id 0, and blocks that never merge keep that id — so graph_join panicked on every real input pair. Pangraph::relabel_in_place re-derives block and node ids as id((salt, old_id)) and renumbers path ids contiguously from an offset; make_disjoint_from applies it to the second graph, leaving the first untouched. This was implemented here rather than as a separate PR because the command is not testable without it.

The command (§4.5). Loads both graphs, checks them (backend availability, non-empty, duplicate genome names across and within the inputs, a warning on mixed circularity), relabels the second, merges, optionally verifies, and writes the result. --verify matches genomes by name rather than by index or position, since neither survives a merge. The superseded bin/merge_two_graphs.rs is removed.

Four integration tests: homologous graphs (all genomes round-trip, and blocks end up shared across the two inputs), appending a single genome, unrelated graphs (the case where colliding small ids survive), and rejection of duplicate genome names.

Still open, tracked in the roadmap: build does not yet reject duplicate FASTA names, and the name-keyed verification is currently private to merge rather than shared with build.