canon-key canonicalizes a collection key by re-keying a native Janet
table by the canonical form of each element/entry. canon-key returns nil
for nil, and a Janet struct can't hold a nil key or value, so a nil set
member / nil map key / nil map value was dropped during canonicalization
— #{nil 1} canonicalized like #{1} and collided as a map key. So
(count {#{nil 1} :a, #{1} :b}) was 1 and (get {#{nil 1} :a} #{1}) was :a.
Box a nested nil before it goes into the table. The marker has to be
value-hashable, not the identity-hashed mutable-table sentinel the
transients use: the canonical struct becomes a long-lived phm key, and
its hash must survive the marshal/snapshot/fork that init-cached relies
on — an unmarshalled table gets a fresh address, so its hash isn't
preserved and the map can't find its own key. An interned keyword hashes
by content. Collision risk is only a real value equal to that exact
keyword, the same negligible class as canon-key's existing set/map struct
aliasing.
The transient sentinel stays a mutable table (it's built and consumed
within one op, never crossing a marshal boundary, so identity hashing is
stable there).
Co-authored-by: Yogthos <yogthos@gmail.com>
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