Bound publication fetches and report site storage

[helly25]

Keep release-site publication proportional to the current retained snapshot, and make whole-site storage growth visible with a 250 MB advisory budget.

AG;DR

• Apply the relevant publication changes from mbo #549: restore coverage-pages with an explicit shallow, no-tags fetch instead of fetching publication history.
• Add storage-report.json with byte and file totals by top-level directory, excluding Git metadata and the report itself. Measure before the retained-tree commit and again after deployment-only artwork, before upload.
• Warn at 250 MB (250,000,000 bytes) and reject payloads above the 9 GB emergency guard. Document GitHub's supported 1 GB site limit separately; the emergency guard is not an operating budget.
• Preserve retained release pages and assets. This repository does not publish coverage, so the port does not add coverage retention or browser loaders. Publication Git history, Actions artifacts, and build caches are unchanged.
• Add regression tests for complete-tree accounting, repeatability, preservation of release bytes, threshold boundaries, and publication ordering. The existing infrastructure test discovery runs these in CI.
• Validation relies on CI, as requested. All current-head CI checks, including the required done gate, passed for 8a32439d2c6c on 2026-10-04. CI results. No local result is used as final validation.
• Standard-library Python only; no new dependency. Update infrastructure guidance and the changelog.
• Targets main independently; no dependency or required merge order with the other rollout PRs. Publication happens through the existing workflow after merge.