Track full-content local search with a performance budget

## Context

The OINK site intentionally keeps native local search on the `summary` index while the optional Ask AI integration is added. Native search must remain independent of any external AI provider.

A full-content index can improve recall, but the current planning estimate is roughly **233–243 KiB gzip per language per version**. Across five versions and two languages, enabling it mechanically would multiply download, parse, and memory cost for users who only need the current documentation.

## Current baseline

- `params.offline_search_index: summary`
- Search metadata and restrained boosts improve the highest-value entry pages.
- Ask AI is an optional tail action; it does not replace or modify native results.
- Historical versions remain independently addressable and must not silently load every other version's index.

## Deferred scope

Do not enable `content` mode as part of the current OINK site-improvement delivery. This issue tracks a separate, evidence-based decision.

## Evaluation requirements

- Measure generated and gzip size for each language and version on an immutable source revision.
- Measure first-open transfer, parse/index time, memory, and repeat-open cache behavior on desktop and representative mobile hardware.
- Compare recall and ranking against the fixed English/Chinese query set used for the summary index.
- Define per-language/per-version budgets and a loading strategy before implementation.
- Preserve explicit load-failure/retry behavior and ensure one failed historical index cannot break latest search.
- Verify keyboard, mobile, and offline behavior in Chromium.

## Acceptance boundary

Adopt full-content indexing only if measured recall gains justify the transfer/runtime cost and all agreed budgets pass. Otherwise retain `summary` and document the result. The 233–243 KiB figure above is a planning estimate, not current-HEAD measurement evidence.

Start by locating the OINK configuration for `params.offline_search_index` and the fixed English/Chinese query set used for summary search. Measure generated and gzip sizes, transfer, parse/index time, memory, cache behavior, recall, ranking, and Chromium keyboard/mobile/offline behavior across the stated languages and versions. Done means agreed budgets and loading safeguards are tested, followed by a documented decision to adopt `content` or retain `summary`.

Written by the indexing model from the issue text.