get_axis sits beside get_var as a plain dict lookup over a projection
build_resolve_ctx makes at frame construction, so the resolver stays a pure
consumer with no ctx dependency. Keyed by binding, matching what the
reference addresses.
Branches before the var lookup: $outfit.SHOES must resolve even when
nothing bound $outfit itself, and must not trip unknown_var on the way
past. Never routes through deref_var_value either - an axis value is not a
ListVar, so the SP2a accessor contract is untouched.
A missing axis renders empty and warns unknown_tag_axis rather than
failing, matching how every other unresolvable reference behaves.
DumiFlex
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