Style merge hardening: cache safety and deterministic coverage

[RtlZeroMemory]

Summary

• hardened the mergeTextStyle() default-base boolean cache by freezing cached entries before storage/return, preventing shared-cache mutation poisoning
• added deterministic cache audit coverage in packages/core/src/renderer/__tests__/style.merge-cache.test.ts (13 tests) for key uniqueness, churn/staleness resistance, tri-state separation, new attrs, and cache hit-rate behavior
• added deterministic fuzz coverage in packages/core/src/widgets/__tests__/style.merge-fuzz.test.ts (11 tests, 1024+ iterations per test, seeded RNG only)
• added inheritance-chain coverage in packages/core/src/widgets/__tests__/style.inheritance.test.ts (16 tests) covering Root->Box->Row->Text precedence, deep chains, and recompute-after-unset behavior
• added utility coverage in packages/core/src/widgets/__tests__/style.utils.test.ts (6 tests) for mergeStyles/styleWhen contracts
• updated docs/guide/text-style-internals.md with cache audit notes and baseline lock metadata

Why

This hardens style merge correctness on the renderer hot path while increasing deterministic regression coverage for cache behavior, merge correctness, and inheritance semantics without adding new style features.

Validation

• npm run lint
• npm run build
• npm run typecheck
• node scripts/run-tests.mjs (2534 passing, 0 failing)