Improve frontend runtime and service performance

[dimaip]

Stack

This is PR 2 of 3 and targets the preceding foundations branch. Merge the stack from the bottom up.

1. Frontend foundations and tooling — #733
2. Frontend runtime and service performance — this PR
3. Performance measurement and evidence harness — #735

What changed

• Replaces TOC polling and random heading IDs with a deterministic, batched heading registry.
• Adds stable accessible heading labels and preserves heading order, levels, anchors, and duplicate handling.
• Adds cached Suspense-based MDX loading with failure recovery and removes repeated per-fragment effects.
• Reduces duplicate calendar/readings queries through explicit freshness policies and stable query keys.
• Defers optional route, menu, editor, and service work until it is needed.
• Adds production-style static delivery with precompressed immutable assets and cache-policy verification.
• Adds a reproducible long-service benchmark, service regression budgets, and field performance marks.
• Records and rejects progressive rendering/chunking variants that did not produce a defensible win.

Why

Long services were paying avoidable polling, repeated subscriptions, duplicate loading, and inactive-feature costs. These changes target measured CPU, render, and delivery bottlenecks while keeping the complete-document contract intact.

Impact and constraints

• Service-worker source and offline corpus behavior are unchanged.
• Dynamic MDX loading, parallel language mode, TOC navigation, print, search, copy/select-all, zoom, theme, and scroll restoration remain covered.
• No custom-select replacement is included.
• Chunking changes were retained only where the service and offline measurements supported them.

Validation

• Deterministic TOC and heading-label unit coverage.
• MDX contract, cache, runtime, query-policy, and static-delivery tests.
• Browser E2E coverage for service, language, TOC, search, offline, and error-boundary behavior.
• Production-build bundle reports and repeated long-service benchmark artifacts.

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Project	Deployment	Actions	Updated (UTC)
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