#1833 Add optional isolated full-TeX chat math rendering

Description

Adds optional XeLaTeX → PDF → filtered SVG rendering for assistant equations, with the shared parser, settings, queue and source-preserving chat integration. The shared core does not render. Rendering starts disabled; the compiler container is built and started explicitly, with no dependencies installed by normal Strata startup.

Refreshed to upstream main `61b3fb5d` and shared math core `253ea8cf`. Compilation has no fixed CPU/wall budget, host transport timeout or TeX browser conversion deadline. Browser-only adapters retain their default deadlines. Cancelled compiler RPCs keep their queue slot until completion and discard the cancelled result. Memory, process, filesystem, input/output and SVG limits remain enabled.

Fresh Windows/Docker Desktop validation: 41 endpoint/security tests, seven valid compiler cases, six malformed/restricted compiler cases, and the complete Chromium, Firefox and Playwright WebKit suites passed. The unbounded recursive TeX macro fixture is explicitly skipped and requires operator cancellation by restarting the worker. Source copy/export/history/model requests remain unchanged; the SVG uses source alt text rather than semantic MathML.

[Current refresh evidence and reproduction](https://github.com/CC-David-CC/Strata-a5500/tree/fba796708ed8f2953eff23eec550e4d094d0e83f/web-math-results/current-main-refresh), including exact image/worker identity, isolation, raw timings and screenshots. Single acceptance-run timings are descriptive; Chromium cold timing includes corpus contention. The archived image is 7.04 GB uncompressed, not an estimated download size.

Addresses [#1769](https://github.com/Niko1221/Strata/issues/1769) and [#1780](https://github.com/Niko1221/Strata/issues/1780). See [setup and adapter contract](https://github.com/CC-David-CC/Strata-a5500/blob/fba796708ed8f2953eff23eec550e4d094d0e83f/docs/WEB_MATH.md).