Controls which conversion tier is used.
Automatically pick the best tier for the input (default).
Force the Tier-1 byte scanner; if it bails, fall back to Tier-2. Testkit-only; not stable API.
Always use the Tier-2 (tl::parse + walk) path, skipping Tier-1.
tl::parse
@type t() :: :auto | :tier2 | :tier1
@spec auto() :: t()
@spec tier1() :: t()
@spec tier2() :: t()