fix(merge): repeated merges, help grouping and release test build

[mmolari]

Follow-up fixes from reviewing #192 and #193, stacked on top of #195 so the reviewed branches stay untouched.

• repeated merges collided on a constant relabeling salt. make_disjoint_from re-derived the right graph's ids as id((1, old_id)). Those ids survive into the merged graph whenever a block or node finds no homologue, so a second merge with that graph on the left mapped the new graph's raw ids straight back onto them, and the disjointness guard fired with a message blaming a 64-bit hash collision. The salt now comes from the path id offset, which strictly increases along a merge chain, and the bounded retry loop is back as the actual guarantee — simplify drops paths without renumbering, so the offset is not monotone under every command. Covered by a unit test and an integration test that appends to an already-merged graph.
• integration tests called sanity_check, which is #[cfg(any(test, debug_assertions))] and therefore missing from the library when integration tests are built in release: cargo clippy --release --all-targets failed at those call sites. The calls were redundant anyway — merge_graphs and merge_run already sanity-check the same graph — so they are gone rather than cfg-guarded.
• --circular, --verify, --no-progress-bar and --guide-tree were listed under the "Alignment" help section, because next_help_heading applies to every argument declared after the flattened GraphMergeParams. That struct is now declared last in both commands. Pre-existing for build; merge had inherited it. CLI reference regenerated.
• merge --verify materialised every genome of both inputs before comparing. It now reconstructs and drops one genome at a time, matching what reconstruct --verify already does. Measured on a 3+1 E. coli merge the peak RSS is unchanged (~1.05 GB either way — the merge itself dominates), so this is consistency rather than a measurable win.

--all-targets in release still fails at #192's own commit; fixing that there would have meant force-pushing three branches.