Installer acts only after the whole script has arrived #312

Part of #89.

Part of #311 (Group B, installer and update). Owner: not assigned. Task base and PR target: develop.

Intended behavior

curl … | bash runs the installer only after bash has received the whole script. A dropped connection produces no partial install and no stopped Runtime.

Current behavior

• scripts/install.sh (1,320 lines) defines its functions and then runs its body at top level, from step 1 "Check this computer" (line 956) to finish_install (line 1320). Only an early if block is read as one compound command.
• With curl -fsSL … | bash, bash runs each line as it arrives. If the connection drops part way, bash runs the part it has, and the pipeline's exit status is bash's own: 0.
• After installer_runtime_control (line 1298), a drop leaves the Runtime stopped with nothing installed.
• After the binary is installed, a drop skips principal-root-upgrade (lines 1310-1312) and setup, so the gateway refuses to start until the user runs the installer again.
• scripts/install-bootstrap-test.py tests a truncated binary download, but not a truncated script.

Implementation notes

• Move every top-level action into main() { … }, and make main "$@" the last line of the file.
• Keep set -euo pipefail and every __STAMP__ placeholder. scripts/release-signer.py (render_installer, lines 376-390) requires each placeholder, and the signature covers the installer bytes (installer_sha256).
• The change reaches users only in the next signed release. The CI update hop installs the previous release with that release's own install.sh, so the hop stays valid.

Done means

• Every action in install.sh runs inside a function, and the last line calls it. Test: a new case in install-bootstrap-test.py feeds the script truncated at each step boundary and after installer_runtime_control through bash. It asserts that no Runtime is stopped, no file is installed, and no data directory is changed. Proof: https://github.com/Elacity/elastos-runtime/blob/aa3d61e4478a1a1a63dbef0dae97c23a0f9f4851/scripts/install-bootstrap-test.py#L844
• Installer stamping still works. Test: the signer tests in release-platform-input-test.py render the new template. Proof: https://github.com/Elacity/elastos-runtime/blob/aa3d61e4478a1a1a63dbef0dae97c23a0f9f4851/scripts/release-platform-input-test.py#L1293
• The macOS update hop in CI passes. Proof: https://github.com/Elacity/elastos-runtime/actions/runs/38241637563/job/114787675815

Verify

python3 scripts/release-platform-input-test.py (runs install-bootstrap-test.py), just ci-local-prepush.

Do this before #313 and #314, which edit the same file.