Minor fixes

[kassane]

Steps

install.sh:

• exit with clear error on unsupported OS/arch combos (macOS x86_64, Alpine arm64, FreeBSD arm64) instead of silently building a broken URL
• add SHA256 checksum verification for all downloaded archives
• add install.ps1 reference in the Windows error message
• replace sed -i with portable mktemp+grep to fix BSD/GNU incompatibility
• replace echo -e and source with POSIX-portable printf and "."
• detect login shell via $SHELL instead of $ZSH_VERSION/$BASH_VERSION
• add set -eu for fail-fast behaviour

install.ps1 (new):

• detect x64 vs arm64 via RuntimeInformation.OSArchitecture
• verify SHA256 with Get-FileHash before extraction
• set LDCUP_DIR and PATH persistently in the user environment
• bootstrap ldc2-latest after install

impl.d:

• fix defaultInstallRoot() called before detectPlatform() causing wrong default path on Windows (currentOS was always .init)
• fix opend toolchain extract path using hardcoded "ldc2-" prefix
• fix compilerVersion never set on the opend-latest early-return path
• fix ref string params on extractTarXZ, extract7z, uninstallCompiler
• extract shellConfigFiles() helper to deduplicate 4 identical blocks
• fix listLDCVersions printing a D array literal; now one tag per line
• use stripLeadingV() instead of .replace("v","") in getCompilerDownloadUrl

app.d:

• fix arg == "--" check (was endsWith, matched "foo--")
• fix leading-v strip using normaliseCompilerSpec() instead of .replace("v","") which stripped every occurrence
• fix "--" index arithmetic being relative to args[1..$]
• change main to return int with try/catch for proper exit codes
• split argument parsing into parseArgs() for testability
• add explicit imports