Share host CI setup as composite actions

Three hosts re-homing their workflows means the same setup repeated: toolchains, SDKs, build
tooling, core checkout. Copied setup drifts, and drift in CI is expensive to debug because
each copy fails differently.

Extract the shared setup into composite actions the host workflows call, so setup is defined
once per platform.

Depends on: #675, #680, #684, since the duplication has to exist before it is worth
extracting.

Steps

• Extract the shared setup per platform into composite actions
• Host workflows call them rather than repeating setup
• One place to bump a toolchain version

Done when

Bumping a toolchain for a platform is a one-line change rather than three.