Port scripts/lib/roadmap_fragments.py (and roadmap_diff/roadmap_merge) to Rust #5902

Operator never-rule: no new Python. roadmap_fragments.py predates the rule and #5901 only adds a fragment-ids subcommand to it (no new .py file, no inline python3 -c). Port the aggregate/changed/adopt/fragment-ids commands to a Rust bin (pv or apr xtask) and retire the .py. Refs #5897.

🤖 Generated with Claude Code

Scope widened (quorum condition 2, #5901 @66719fd864; receipt handoff/triage-quorum-5901-python-growth.md)

The port retires ALL Python that #5901 grows, not only the .py:

• scripts/lib/roadmap_fragments.py +56/-8 (+48 net): --base-file mode, _emit_generated, fragment-ids subcommand.
• scripts/ci/fat_driver.py +8: the setup_workspace subprocess.run(["python3", roadmap_fragments.py, "aggregate", ...]) call.
• 4 new python3 call sites in the .sh guards: check_roadmap_diff_additive.sh (aggregate, +1), check_roadmap_fragment_required.sh (fragment-ids and aggregate --check, +2), plus the Makefile roadmap-aggregate(-check) targets (existing calls, --base-file added).
• ~11 lines added inside the pre-existing embedded Python heredoc of check_roadmap_completion_is_cited.sh.
  Done when: no python3 reference to roadmap_fragments.py remains in scripts/, Makefile or ci.yml, and the .py is deleted.
  Until it lands: no further python3 -c, new heredoc or new .py on this path (quorum condition 3).