Documents the pure C11 loader (tiny_obj_c + tobj_tess, merged in #430) in the README and refreshes stale content.

Changes

• Co-equal mains. The intro now presents C++11 (tiny_obj_loader.h) and C11 (tiny_obj_c.{c,h} + tobj_tess.{c,h}) as two co-equal, independently usable implementations.
• New "Pure C11 loader" section covering its properties (freestanding / dependency-free, dual float+double precision, robust tessellation, faces/lines/points/free-form primitives, full materials, optional multithreading/SIMD/mmap), a usage example, and build notes (CMake or just compile the two .c files).
• What's new entry (19 Jun 2026) and a C11 compiler requirement.
• Removed stale content: v0.9.x branch notes, the 2016 v1.0.0 changelog line, the "Old version" subsection, and the external tinyobjloader-c (C99) pointer — the latter is superseded by the built-in C11 loader.

Docs-only; no code changes.

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