Cache `uses_phys_key` as a `QTerm` field

[oameye]

_uses_phys_key(term) re-runs an O(n²) phys_ops × phys_ops scan against ne on every isequal/hash (qterm.jl:28), and since QTerm is the dict key for every QAdd, this fires on every insertion and lookup.

Cache it as a uses_phys_key::Bool field set once in _term_key (after _canonical_ne) and read by isequal/hash. Tried briefly and reverted, re-attempt with a benchmark to confirm the win and that the extra field doesn't regress construction allocations.