refactor(desktop): adopt Proton PR #129 runtime

[Milky2018]

Summary

• pin Proton PR #129 at dc9a107282eedb2c5329dba15fbe7d4d8bbf5ec6 and resolve its source modules from the root workspace
• migrate the desktop project configuration, development launcher, and platform packagers to the source-built private Proton runtime contract
• source the CEF runtime from the immutable Proton store and remove the legacy libproton, .proton/runtime.json, and PROTON_NATIVE_DIST contract
• package the source-matched helper on macOS, Linux, and Windows, including a macOS 12.0 deployment-target compiler identity for cached native builds

Validation

• moon -C desktop check --target native --diagnostic-limit 200
• moon -C desktop test package/internal/packaging --target native (11/11)
• moon -C desktop test package/macos --target native (5/5)
• moon -C desktop run --target native package/macos -- --release --target app --no-open
• codesign --verify --deep --strict --verbose=2 desktop/dist/SeekMoon.app
• verified the arm64 host and all five CEF helpers use macOS 12.0 as their minimum deployment target
• verified the app contains no legacy libproton.dylib, proton.dll, or proton.lib
• launched SeekMoon.app, confirmed it remained running, and terminated it cleanly
• just build

Current integration gate results

• just check currently stops on 54 implicit_impl_as_method warnings promoted by --deny-warn; all reported locations are outside this diff
• just test currently reports 3504/3513 passing; the 9 failures are in agent_tool edit, multi-edit, remove, and write tests where temporary Moon workspace/check output enters the expected result, outside the Proton and packaging paths touched here

Dependency

This draft intentionally tracks the unpublished source and contracts from moonbit-community/proton#129.