Mirror the zenai provider pattern: the enum's declaration order is the
single source of truth for the .auto cascade, asserted at comptime against
api_engines (auto first, one table entry per API engine in tag order,
duckduckgo last with no entry). The enum reorders brave before tavily to
match the table's actual preference — persisted settings are unaffected
(ZON stores tag names).
Each table entry now carries a .keyless field; the hardcoded keenable
keyless rung, searchEngineKeyless, and searchExplicit's comptime gate all
derive from it, engineIndex collapses to arithmetic, and apiSearch takes a
typed ?[]const u8 key instead of anytype.
Adrià Arrufat
12ecf27fb1