scalar-replace already folds non-escaping const-key map literals
((:k {:k a ..}) -> a, and drops a let-bound map that doesn't escape).
Extend the same fold to record constructors: a (->Rec a b c) is a
positional struct whose declared field order lives in the record-shapes
registry, so a field read on a non-escaping ctor folds to the matching
positional arg and the allocation disappears.
Direct form (:field (->Rec ..)) and the let-bound form both handled,
threaded through run-passes via a per-unit shape registry (new
jolt.host/record-shapes accessor). Soundness: ctor args must be pure
(duplicated/discarded like map vals), arg count must equal the field
count, and only declared-field reads fold — a record answers the virtual
:jolt/deftype key with its type tag and any other key with nil, neither
of which is a positional arg, so those keep the allocation. pure? now
treats a record ctor of pure args as pure, so nested records (a Ray
holding a Vec3) fold bottom-up.
Allocation-bound microbench (non-escaping record built + field-read in a
hot loop): 69.6s -> 2.4s, landing on the no-record arithmetic baseline.
The ray tracer is unchanged — its vec3 results escape (returned/stored
each op), so they genuinely allocate; that's a separate problem.
Co-authored-by: Yogthos <yogthos@gmail.com>
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