MODERNIZE: enable uv/pipx install with runtime sudo elevation

[ventusff]

Summary

Make a user-level install (e.g. uv tool install bt-dualboot, pipx install bt-dualboot) the recommended install path. The tool now self-elevates via sudo at runtime when a root-needing action is requested, and sudo prompts for the password on the TTY. Pass --no-elevate to opt out and run as root yourself.

Also includes a drive-by fix for case-insensitive Windows registry path resolution, surfaced while testing on a real dual-boot machine.

Usage

$ uv tool install bt-dualboot          # or: pipx install bt-dualboot — no sudo
$ bt-dualboot --sync-all --no-backup          # no sudo required
[bt-dualboot] root privileges required; re-executing under sudo (use --no-elevate to disable)
[sudo] password for user: ********

Syncing...
==========
 [C2:9E:1D:E2:3D:A5] Keyboard K380
...done

What changed

Runtime privilege elevation (6975ad1)

• New bt_dualboot/cli/privilege.py with action_requires_root(opts) and elevate_via_sudo().
• cli/app.py calls elevate_via_sudo() from main() for -l, --sync, --sync-all. New --no-elevate flag opts out.
• Re-exec prefers the entry script when it's executable (e.g. ~/.local/bin/bt-dualboot); falls back to python -m bt_dualboot otherwise. The DEBUG env var is preserved across the sudo boundary via sudo --preserve-env=DEBUG.
• README install/usage rewritten — sudo pip install is no longer recommended; uv tool install and pipx install are documented.

Case-insensitive registry path resolution (b990db1)

• New resolve_path_ci(base, relative) walks each segment under base, preferring exact match and falling back to a case-insensitive listdir lookup.
• Used by WindowsRegistry._registry_file() and by locate_windows_mount_points() (replacing glob.glob, which is case-sensitive on Linux).
• Fixes openHive(...) failed: No such file or directory on Windows installs whose hive is on disk as system (lower-case) rather than SYSTEM. NTFS is case-insensitive in Windows itself, but Linux NTFS drivers (ntfs-3g, ntfs3) are case-sensitive by default.

Test plan

• pytest tests/ — 48 passed (was 42; +6 new tests covering the case-insensitive resolver)
• As a regular user: bt-dualboot --version, --list-win-mounts, -h — no sudo prompt
• As a regular user: bt-dualboot -l — re-execs under sudo, prompts for password, lists devices
• As a regular user: bt-dualboot --sync-all --dry-run --no-backup — DRY RUN output after sudo prompt
• As root: sudo bt-dualboot -l — runs without re-elevation (early return when euid == 0)
• --no-elevate exits with a clear error when the action would need root
• Verified end-to-end on a Windows 10 install whose hive is on disk as system (lower-case) — failed with openHive(...) failed: No such file or directory before this PR; resolves correctly now
• Maintainer to verify on a Windows install with upper-case SYSTEM — exact match is tried first, so this path should be unchanged

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