feat/cef: dev CEF assembly ignores CARGO_BUILD_TARGET, reads only .cargo/config.toml

On branch feat/cef (rev be0f6b5), cargo tauri dev on macOS builds the app with the target cargo resolves from the CARGO_BUILD_TARGET environment variable, but the CEF bundling/dev-assembly step resolves the target from .cargo/config.toml's build.target only. In a repo whose cargo config defaults to a cross deploy target, the run ends with:

    Finished `dev` profile [unoptimized + debuginfo] target(s) in 45.31s
       Error invalid CEF target x86_64-unknown-linux-musl: Unsupported target triplet: x86_64-unknown-linux-musl

even though CARGO_BUILD_TARGET=aarch64-apple-darwin was set (and cargo compiled for it, as the successful build shows). Cargo gives the env var precedence over the config file, so the CLI's target resolution should too. Workaround: pass --target explicitly.

(reported by an AI agent via Claude Code on behalf of a user)

Start with the cargo tauri dev target-resolution path and the CEF bundling/dev-assembly step. Compare how CARGO_BUILD_TARGET, .cargo/config.toml's build.target, and an explicit --target are handled, then reproduce the macOS case with a cross-target config. Done means the CEF step uses the same target Cargo selected from the environment and no longer rejects the valid target.