Cross-platform compatibility fixes for Windows and macOS

[mmacferrin]

Summary

Fixes a set of cross-platform compatibility issues so IVERT runs on modern Windows and macOS as well as Linux. Each fix preserves existing Linux behavior. No functional change on Linux.

Fixes

• Config path resolution (configfile.py) — Path detection was gated on sys.platform: the Windows branch only recognized values containing a backslash, but the .ini files use forward slashes (e.g. ~/.ivert). On Windows those values fell through unhandled, leaving ~ unexpanded. Replaced the two platform-specific branches with one platform-agnostic branch (treats a value as a path if it contains ~, /, or \) plus an _is_absolute_path() helper that recognizes Unix, Windows drive, and UNC roots regardless of OS. On Windows ~/.ivert now resolves to C:\Users\<user>\.ivert.

• signal.SIGKILL guard (validate_dem.py) — signal.SIGKILL does not exist on Windows, so abs(exitcode) == abs(signal.SIGKILL) raised AttributeError whenever a validation subprocess exited non-zero. Look the attribute up with getattr and skip the OOM-killer recovery branch when it is absent (Windows has no SIGKILL / OOM-killer); a failed worker now surfaces as the intended RuntimeError.

• Cross-platform terminal colors (bcolors.py) — Raw ANSI escape codes render on Linux/macOS but appear as literal garbage on Windows consoles, where virtual-terminal processing is off by default. Enable it on import via colorama.just_fix_windows_console() (no-op elsewhere; only sets the console-mode flag, does not wrap sys.stdout, so it does not interfere with loggerproc.Logger's stdout redirection). colorama added as a Windows-only dependency; the import is defensive so Linux/macOS degrade to native ANSI.

• Multiprocessing start method (validate_dem.py, validate_dem_collection.py) — set_start_method("spawn") raises RuntimeError if the start method was already set. Pass force=True so re-invocation cannot crash.

• Explicit UTF-8 encoding — Text-mode open() defaults to the platform locale encoding (e.g. cp1252 on Windows). Pass encoding="utf-8" on all text-mode opens (empty-result markers, summary stats, user config, photon-tile lists, the AWS datasource probe, and the process logger). Binary opens are unchanged.

Testing

Verified on Linux: config paths still resolve to the correct absolute home-directory locations; the SIGKILL decision logic was unit-checked across POSIX/Windows exit-code cases; bcolors imports and renders (VT-enable is a no-op on Linux and degrades gracefully if colorama is absent); set_start_method(force=True) is idempotent; and no text-mode open() without an encoding remains. ruff check and ruff format pass on all changed files.

Not covered here (requires real Windows/macOS hardware): the shared-memory + spawn parallel-validation path in validate_dem.py — Windows frees named SharedMemory segments when the last handle closes, unlike Linux /dev/shm, so that flow should be exercised directly on the target platform.

[mmacferrin]

Issue #10 progress (refs #10 — does not close it)

This PR is part of the ongoing cross-platform work for #10. It lands the code-level fixes; the issue stays open pending on-hardware testing.

Done in this PR (5 fixes, Linux behavior unchanged):

• Config ~/path resolution works on Windows (configfile.py no longer gates path detection on sys.platform)
• signal.SIGKILL guarded so validate_dem.py no longer raises AttributeError on Windows
• Terminal colors render on Windows consoles (colorama.just_fix_windows_console()), colorama added as a Windows-only dep
• mp.set_start_method("spawn", force=True) so re-invocation can't raise RuntimeError
• Explicit encoding="utf-8" on all text-mode open() calls (avoids cp1252 on Windows)

Still open under #10 (not in this PR):

• Exercise the shared-memory + spawn parallel-validation path on real Windows/macOS — Windows frees named SharedMemory segments when the last handle closes (unlike Linux /dev/shm), so this needs to be run on the target platform rather than fixed blind
• Manual testing: macOS (Intel + Apple Silicon), native Windows / WSL2, CentOS/RHEL/Rocky
• Cross-platform GDAL/h5py install differences; AWS credential-path variations
• CI/CD matrix across platforms; document supported platforms

Reminder: keep closing keywords away from #10 in the eventual merge commit/PR — GitHub ignores negation and auto-closed it once already.