coro: complete ProgramIR host event and stdlib transport core

[cpunion]

Summary

• hard-cut coroutine emission over a frozen ProgramIR contract, including exact HostOp call shapes and one shared park lowering path
• unify worker, timer, keyed wait, cancellation, channel/select, and host I/O under the executor event model
• add freestanding WebAssembly host-operation support for wall/monotonic time, files, DNS, TCP, cancellation, and standard-library-style synchronous calls
• close exact nil/bounds fault lowering gaps and keep native fleet scheduling isolated from foreign blocking work
• add architecture debt gates and split LLVM integration E2Es into bounded independent CI lanes

Validation

• 310 compiler coroutine tests pass (go test ./cl -run ^TestCoro)
• target-neutral runtime tests and race tests pass
• all 8 native linked/fleet E2Es pass
• fresh wasm-unknown and wasip2 runs pass for HostOp network, standard-library file I/O, time.Sleep, and standard-library TCP/DNS
• architecture monotonicity gate passes; direct plan authority snapshot is reduced from 354 to 352

This is an internal cpunion staging PR onto llvm-coro; full repository workflows remain reserved for the eventual PR targeting main. Related proposal: xgo-dev#1546

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.