windows: integrate OS and standard-library support (R6, depends on xgo-dev/llgo#2405)

[cpunion]

Integration PR for R6 of xgo-dev#2325. This PR targets the R5 fork branch so the complete Windows OS and standard-library stage can run CI without opening the final upstream PR before its dependency merges.

Scope:

• reuse and bridge the pinned Go Windows runtime and standard-library implementations
• add files, processes, console signals, testing hooks, CPU profiling, and IOCP networking
• run Windows language, extension, test/std, demo, symbol-coverage, and GOROOT compatibility coverage
• retain macOS and Linux behavior and avoid adding hosted Windows code to other targets

Current native validation uses Go 1.26.5 on Windows ARM64 in Parallels. All 32 GOROOT coverage shards have been exercised. The only remaining receiver-diagnostic failure was extracted into the independent upstream PR xgo-dev#2417 and is intentionally not duplicated here.

This remains draft while CI and the R6 diff audit are completed. The final upstream R6 PR will be based on the merged R5 boundary.