One XDG base-directory resolver; exhaustive transport walks

Two routines resolved the same XDG base directories by two different rules:

• the xdg: grant sigil went through path::sigil::resolve_xdg;
• the binary’s own config/data loaders (rc file, history, plugin search, exit-hint overrides) went through path::config::config_base / data_base.

The two disagreed on the edges:

• config_base returned $XDG_CONFIG_HOME verbatim even when relative, read the process $HOME directly, and appended a %APPDATA% fallback;
• the sigil resolver honoured $XDG_*_HOME only when absolute (the XDG spec’s rule that relative values are ignored) and fell back to the home-joined Linux defaults on every platform.

So a grant naming xdg:config and the rc loader could point a user at two different directories.

Resolution: a single resolver, path::basedir (XdgKind, resolve_xdg), owns the rule. XdgKind::default_suffix makes each kind carry its own Linux default; resolve_xdg reads the override only when absolute, else joins the default onto home. path::sigil and path::config both defer to it — config_base / data_base / xdg_config_subpath / xdg_data_subpath are now thin wrappers that keep a base only when it resolves to an absolute path. The policy is XDG-everywhere including macOS (xdg:config is ~/.config, no ~/Library/Application Support), matching how cross-platform CLI tools and dotfiles use XDG. The relative-$XDG_*_HOME-verbatim behaviour and the %APPDATA% fallback are dropped; %HOMEDRIVE%%HOMEPATH% is not added — $HOME/%USERPROFILE% resolution stays in path::home. The legacy home_dot (~/.ralrc) convention is unchanged: a dotfile, separate from XDG.

The same edit hardened the serde mirror in core/src/serial.rs. The three hand-written walks that mirror the runtime Value to SerialValue now all match their value type exhaustively. from_runtime already did; the handle-sanitiser value_carries_handle (over Value) and the dependency collector collect_scope_deps (over SerialValue) ended in _ => catch-alls, so a future container-shaped variant would compile while being silently not descended — a spurious snapshot failure or a missing scope-dependency edge in build_arcs. Listing every variant forces both walks to be reconsidered when a variant is added. Behaviour is unchanged: closures (Lambda / Block) are not descended for handles (their captured env is sanitised separately by intern_scope) and contribute their captured.scopes as dependencies; containers recurse; scalars do nothing.

Realised in core/src/path/basedir.rs, core/src/path/sigil.rs, core/src/path/config.rs, and core/src/serial.rs. See capabilities and transport.