rootless-support #78

@intFromBool returns u1 in Zig 0.16, so adding three u1 values
overflows on the second addition (1 + 1 > u1 max). Widen to u8
with @as before adding.

Add a global --root/-r flag and MERE_ROOT environment variable to set
the root prefix path for all mere operations. This enables fully
rootless package management by pointing the store, profiles, gc-roots,
and config to a user-owned directory tree.
Precedence: --root flag > MERE_ROOT env > default (/)
The internal plumbing (Context.root_path) already supported arbitrary
roots — this change exposes it externally.
Closes #67