Finish V8 15.2 runtime integration

[alva-bot01]

Summary

The V8 15.2 build itself succeeded on all four targets in run 33655144716, but the generated commit could not link or pass the release tests. This fixes the remaining integration issues:

• Download architecture-independent artifacts directly into deps/ so V8 15.2 headers replace the stale V8 13.6 headers instead of landing under deps/include/include/.
• Restore the already-built Chromium libc++ headers to their intended paths and include Chromium's __config_site and assertion handler.
• Commit generated deps/*_*/cgo.go files and link every split V8 archive plus libc++-cr.a and libc++abi-cr.a.
• Use Clang 21 and hardened libc++ settings for the cgo bridge and CI.
• Migrate the V8 APIs that became deprecated in 15.2 and add the standard includes required after Chromium removed transitive libc++ includes.
• Preserve the corrected artifact layout for future weekly upgrades.

Verification

• actionlint
• python3 -m py_compile deps/build.py deps/build_common.py deps/update_cgo.py
• CC=clang-21 CXX=clang++-21 go vet ./... with local dependency workspace
• CC=clang-21 CXX=clang++-21 go test -a -count=1 ./... with V8 15.2 libraries and headers
• Clang 21 leakcheck build and execution: PASS
• git diff --check

Previous run

All four platform builds, artifact upload, generated commit, and go.mod sync succeeded in run 33655144716. Only its release test failed because the generated link/header configuration was not committed correctly.