PROD-2350: match page zones and template sections by ID, not position/name

[5PK]

Summary

• Fixes PROD-2350: page zones were mapped to target template sections by array position, so a page push silently misplaced content whenever the source and target templates listed their sections in a different order.
• Adds SectionMapper (same pattern as ModelMapper/ContainerMapper) to persist source↔target pageItemTemplateID pairs, seeded from the confirmed savePageTemplate response and backfilled when an up-to-date template is skipped.
• translateZoneNames, the template-pusher section payload builder, and TemplateMapper.hasTemplateChanged all now prefer this ID mapping — falling back to a direct name match (and finally position, for translateZoneNames) only when no mapping exists yet (bootstrap / brand-new section).
• Also closes two related latent bugs found along the way: renaming a section on the source could cause the template pusher to create a duplicate section on the target instead of updating the existing one, and the old reference-name-sort comparison in hasTemplateChanged could produce a false "unchanged" verdict if two sections traded reference names and item orders at once.

Test plan

• npx tsc --noEmit — clean
• npx jest — 104 suites / 1870 tests pass
• New regression test reproduces the exact PROD-2350 repro (reordered-but-same-named sections) and confirms correct zone placement
• New tests cover: SectionMapper CRUD, template-pusher section-ID seeding/backfill/bootstrap-fallback/rename handling, and the hasTemplateChanged false-negative fix

🤖 Generated with Claude Code