Review: runtime G/M/P pthread backend

Overall this is a solid, well-structured change. The runtime side is clean and, if anything, a small improvement: go lowering now emits a single runtime.NewProc call, and newproc1 does one combined AllocRoot for the G/M/P context. The runtimeContext lifecycle (alloc → mexit → FreeRoot), the mutually-exclusive arg-free paths, and the EnterLocalContext/LeaveLocalContext pairing are internally consistent. The switch of the FuncForPC/symtab caches to atomic pointer publication is the correct call now that goroutines run on real threads. No blocking memory-safety defect found.

The actionable items below are all compile-time performance and maintainability concerns — none block correctness. Inline comments carry the specifics; a couple of non-inline notes follow.

Non-inline notes (low severity / nits):

• runtime/internal/runtime/os_pthread.go header still reads Copyright (c) 2024 (carried over from the z_thread.go rename); every other new/rewritten file in this PR uses 2026.
• runtime/internal/runtime/os_pthread.go newosproc ignores the return of attr.Destroy(). Low risk and conventional (the create-critical returns — Init/SetDetached/SetStackSize/Create — are all checked), noting only for completeness.
• internal/build/main_module.go defineLibraryRuntimeInit: the doc block describes only the global_ctors registration but the function now also conditionally emits the enterLibraryLocalContext call when Prog.NeedsLocalContext() is true. Worth extending the comment, since that call is what lets exported Go calls on a C constructor thread inherit a context.