[0.2.0] Unified compiled-page and surface cache budget partition

[cursor]

Parent: #356

Harvest from gh-143 PDFPageCacheBudget / tst_pagesurfacebudgettest.cpp::partitionsSingleLimit.

gh-143 split a single byte limit 50/50 between compiled pages and rendered surfaces under setCacheLimit(). On dev, PageSurfaceCoordinator owns maxAdmittedBytes but there is no unified partition with the document session compile cache.

Acceptance

• One byte-budget authority partitions between compile cache and admitted surfaces
• Combined resident cost cannot exceed the configured limit
• Unit test for partition math and admission under shared pressure

Reference

• [Interaction quality] Render selections, guides, and preflight markers as independent overlays #143 pdfpagesurfacecache_p.h PDFPageCacheBudget
• docs/handoffs/0.2.0-gh143-parity-checklist.md