fix: reject pnpm 11 on arm64 musl Linux

[cytle]

Closes pnpm/tasks#49.

Both installers now reject pnpm 11 on Linux ARM64 musl before requesting the executable. They explain that this target has no working binary and suggest either npm install -g pnpm@<resolved-version> or pnpm 12. pnpm 10, pnpm 12, ARM64 glibc, and x64 musl retain their existing mappings.

install.sh and get-pnpm previously accepted this target, requesting a pnpm 11 artifact that either crashes or is no longer published. The TypeScript CLI and exported downloadPnpmExecutable API use the same guard. An optional Target.version preserves exact-version guidance without breaking callers that supply only the major.

Validation

• Regression tests failed before the fix and pass after it. They simulate the host platform and assert that no executable download is attempted, including for pnpm 11 prereleases.
• get-pnpm: pnpm test passed (TypeScript compilation and 78 tests, no skips).
• test/target.sh, test/resolve-version.sh, and test/shadowing.sh passed under sh, bash, and dash on macOS.
• ShellCheck 0.11.0 passed for the changed Shell files with the bash, dash, ksh, and sh dialects; git diff --check passed.
• The existing test/verification.sh install.sh could not finish locally: its registry fixture did not finish starting within its 10-second wait. CI will exercise this separately. No native ARM64 musl binary was run locally.

Maintainer action before merge

Changing install.sh makes its signed checksum stale. A maintainer with the pnpm release key needs to run ./_update.sh to regenerate SHASUMS256.txt and its signature. These files are left unchanged; the integrity check is expected to fail until they are re-signed, as documented by the repository's npm-key-rotation workflow.

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