Port the setup script to Linux

[saty-a]

linux/setup.sh is currently a stub that prints a pointer to the official guide and exits 1. This issue tracks bringing it to parity with the Windows script.

What parity means is specified in CONTRIBUTING.md → The porting contract — same flag surface (--verify-only, --skip-android, --install-root, --precache), same exit codes (0 ok / 1 missing or failed / 2 installed but flutter doctor found toolchain issues), idempotent steps that check before acting, and environment changes written to the shell profile behind a marker-delimited block instead of the Windows registry.

Rough shape of the work

• Resolve and install the latest stable Flutter SDK (releases JSON, Linux archive, SHA-256 verified)
• Install Temurin JDK 17 (Adoptium API), set JAVA_HOME
• Install Android cmdline-tools into cmdline-tools/latest, then platform-tools, platforms;android-36, build-tools;36.0.0
• Accept Android SDK licenses non-interactively
• Write PATH / ANDROID_HOME / JAVA_HOME into the shell profile idempotently (bash and zsh)
• Decide and document how to handle the distro-package prerequisites Flutter needs (curl, unzip, git, xz-utils, libglu1-mesa) — including the case where the user has no sudo
• --verify-only mode that changes nothing
• Run flutter doctor and map its result to the exit-code contract

Partial work is welcome — comment here saying which step and which distro you're taking.

[saty-a]

Done in 5c82c91 — linux/setup.sh is now a full implementation.

It matches the other two scripts on flags, exit codes, idempotency, summary vocabulary and logging, so the porting contract is now a consistency spec rather than a to-do.

One honest limitation up front: x86_64 only, and the script refuses arm64 rather than guessing. Google publishes no prebuilt arm64 Linux Flutter SDK — there is no arm64 entry in releases_linux.json at all — so on aarch64 it exits 1 and prints the git-checkout route plus flutter#109609. Installing an x86_64 SDK that cannot execute would have been worse than failing.

Things that genuinely differed from the macOS port:

• Flutter ships .tar.xz on Linux (.zip on macOS/Windows), so there is a new extract_tarxz using tar -xJf — and xz is checked up front rather than letting tar fail mid-run.
• Temurin Linux uses the plain bin/java layout, not macOS's Contents/Home bundle.
• Checksums use sha256sum/sha1sum. Minimal Linux images ship coreutils but often have no shasum — a clean ubuntu:24.04 has sha256sum and not shasum, which is the reverse of macOS. shasum is kept as a fallback.
• stat -c replaces BSD stat -f.
• cmdline-tools is one architecture-neutral archive on Linux, unlike the per-arch mac builds.
• Prerequisites need root, so they are never installed. A clean ubuntu:24.04 has no curl, unzip, xz or git; the script detects what is missing and prints the command for your distro, derived from /etc/os-release.
• The JVM truststore is deliberately not overridden. That is a Windows-only fix (the JVM ignores the Windows cert store); on Linux the JDK's bundled cacerts is already the right store, so overriding it would only break TLS that works.

Tested in an x86_64 container (Ubuntu 24.04, bash 5.2) as a non-root user:

Scenario	Exit	Result
Fresh install	0	[✓] Flutter (3.44.9), [✓] Android toolchain (SDK 36.0.0)
Re-run	0	every component Already installed
--verify-only	0	all present, read-only
arm64 container	1	refuses with the git-checkout guidance
No prerequisites (Ubuntu)	1	sudo apt-get install -y curl unzip xz-utils
No prerequisites (Fedora)	1	sudo dnf install -y unzip

Exactly one profile block after repeated runs, written to ~/.profile. The missing Chrome and Linux toolchain (desktop GTK) validators are correctly scoped as out-of-scope, so a bare container still exits 0 — this script sets up Android builds, not desktop or web targets.

Two things I could not test and would value reports on (#7): a real x86_64 machine rather than an emulated container, and distros beyond Ubuntu/Fedora — the Arch/openSUSE/Alpine package-name hints are reasoned, not verified.