PR write-set: no PR writes docs/roadmaps/roadmap.yaml or the README census; main regenerates both (operator P4) #4417

Operator doctrine P4 (2026-09-25, relayed by the cop): a PR must not write docs/roadmaps/roadmap.yaml or the README census. Main regenerates both.

Today: scripts/check_row_pr_write_set.sh (G-11) refuses these writes only for ROW PRs on agent/<id> branches. Every other PR can hand-edit the aggregate and the README counts. scripts/check_roadmap_fragment_required.sh rule 2 goes further and requires each PR to regenerate the aggregate, which directly conflicts with P4.

Change:

1. New guard scripts/check_pr_generated_write_set.sh. guard_tree.sh picks it up automatically, so ci.yml needs no edit.
   • Over merge-base..head in pull_request or merge_group shape, it goes RED if the diff writes docs/roadmaps/roadmap.yaml.
   • It also goes RED if the diff changes a README census line: a CONTRACT_COUNT marker, or a count claim matched by the check_readme_claims.sh extractors.
   • The only exemption is the single-writer regen path. Its CLI contract is to be agreed with the owner of pmat#1370 (pmat roadmap sync as the sole writer, plus sync --check).
   • Ships a must-match / must-not-match case table under --self-test.
2. check_roadmap_fragment_required.sh: in PR shape, the aggregate may lag its fragments; --check exactness moves to main.

Merges into batch/0.70.0; fold owner is aprender-59. Related: #3650, #3709, pmat#1370.