build: Hash the cancellation pass and accept version-file comments
Include llvm-cancellation-lowering.cpp in CODEGEN_SRCS_HASH so
incremental builds cannot reuse stale object-cache output after the
pass changes; teach the documentation version-file parser to skip
comments and blank lines (the staging pins carry explanatory comments);
define the library binding the C-interop manual's cancellation example
references.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015KLwCBAR64r3cTdfaC5krU