fix: preserve SSE state in x86_64 bthread context switch

[Spicy-cream]

Follow-up to #21.

Changes since the previous internal review:

• Add a standalone bthread_context_unittest.cpp without modifying existing UTs.
• Verify preservation of XMM6-XMM15 across context switches.
• Replace TaskTracer magic register indices with named frame-layout constants.
• Guard the optional RSP slot constant for older libunwind versions.

Verification:

• The complete private CI matrix passed.
• The new XMM context test passed with Clang on Linux x86_64.