fix(docs): restore canonical Umami analytics

[jeremi]

Summary

• restore Umami on the canonical released documentation with the Registry Docs website ID stored as a reproducible source input
• keep analytics disabled on /dev and immutable /v/<version> archives, and fail release archive builds when that boundary is violated
• add an accessible Privacy page describing the docs analytics events, hosting data, retention, and visitor rights

Why

The hermetic release archive builder clears deployment-provided PUBLIC_UMAMI_* values. RegistryHead.astro previously required the website ID from that environment, so the canonical archive was published without the tracker even though the public website remained correctly configured.

This change preserves hermetic release builds while binding the canonical docs tree to the existing Umami property:

• website ID: 0a8aa090-83c5-4207-8c90-9fcc1e50bb78
• script: https://stats.registrystack.org/script.js
• domain: docs.registrystack.org

Verification

• npm test: 450 tests passed
• npm run check: passed, including all tutorial dry runs, accessibility, SEO, LLM output, and 18,829 internal links and assets
• DOCS_DOCSET=v0.25.0 npm run build:archive: passed against the real release path
• release artifact verification: canonical root contains exactly the expected tracker; versioned archive contains no analytics
• final privacy edit: content, Markdown, Vale, and Astro build checks passed

The existing unrelated Astro deprecation hint, three Vale suggestions, and OpenAPI lint warnings remain non-blocking.

Review and rollout

The Privacy page is Tier C under the docs guidance and requires maintainer sign-off before merge. Its Umami details were checked against the official FAQ and metric definitions.

Because canonical docs are promoted from immutable release archives, merging this PR does not rewrite the currently published archive. Tracking resumes on docs.registrystack.org when the next Registry Stack release archive containing this fix is published.

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