The gap that let the shell-theme breadbar ship half-verified: a builtin
theme.toml can parse in isolation but fail `resolve_theme` at runtime (a
slot name the manifest no longer knows, a field rename, a broken
`extends`), and `load()` silently falls back to liquid-motion. breadbar
has no tracing subscriber so that fallback's `warn!` goes nowhere — the
shell just quietly ignores `active = "spotlight"`.
Three tests over `builtin::ALL` (not a hand-maintained id list):
- every_builtin_theme_resolves: load_named() succeeds, id/name match, has
window anchors
- active_theme_id_env_selects_each_builtin_and_never_falls_back: BREAD_SHELL_THEME
set to each id → load() returns that id, not the fallback
- unknown_theme_id_falls_back_to_liquid_motion: the fallback still works
Adding a fifth builtin now needs its theme.toml to be loadable, not just
parseable.
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