feat: soft-launch comparison route

[shaug]

TL;DR

Enables the current Washington primary comparison experience at its direct URL while keeping every existing public navigation and About surface unchanged.

Summary

• adds a strict current-election-only hosting preview declaration that stages the already-audited comparison experience without changing its release promotion policy
• publishes /e/wa-2026-primary/compare/ with canonical slash behavior while preserving unknown and historical comparison-path 404s
• keeps the guide, Sources, About, archive, and 404 surfaces free of inbound Compare links
• requires the preview route in deployment verification so a missing page and manifest entry cannot pass together
• documents the soft-launch hosting semantics and adds build, stage, verification, preservation, and regression coverage

Validation

• uv run pytest tests/test_comparisons.py tests/test_compare_rendering.py tests/test_hosting.py tests/test_hosting_models.py tests/test_release.py -ra (77 passed)
• make check (430 Python tests and 123 JavaScript tests passed; Ruff, Pyright, inventory, sources, and reproducibility checks passed)
• exact-head release build, hosting stage, and hosting verification (1 election, 15 assets)
• local Wrangler desktop, 390px mobile, no-JavaScript, redirect, 404, accessibility, overflow, and direct-link crawl smoke checks
• clean repository-owned review on 20f36792177fd070d1ecba5602f9933fce62ea15

Production deployment and exact-SHA live verification remain intentionally post-merge before issue closure.

Tickets

Refs #173