Removes the {:r :g :b} hardcoding. ANY constant key set is now a shape:
- inference: a struct type from a map LITERAL carries :shape (its canonical
str-sorted key vector — completeness); joins/access-inferred structs lack
it, so they never get a bare index. The literal node and lookup subjects
carry the shape; the back end derives the index from it.
- backend: emit-map turns any shape-tagged const-key map into a shape tuple;
emit-kw-lookup reads the field by bare index when the complete shape is
proven, else by the value's own descriptor (so a shape-rec whose :shape was
dropped by a join still reads correctly).
- runtime: core-get and core-assoc handle shape-recs.
Status: CORRECT for direct field access, container round-trips, and assoc
(minimal repros pass). NOT yet complete — the full ray tracer still hits an
uncovered path (a shape-rec reaching a map op without coverage: keys/vals/
count/seq/equality/print/jolt-call/dissoc/contains?/the interpreter's
coll-lookup all still need shape-rec branches). And the perf win needs
COMPLETENESS PRESERVATION through joins/containers (merge-fields/cap drop
:shape today, so nested vec3 access falls to the descriptor path, slower than
a struct get) — without it the general version is slower than the vec3
prototype.
All behind JOLT_SHAPE (off by default). Gate green with the flag off, suite
4718. This preserves the general design; the transparency layer + completeness
preservation are the remaining multi-session work.
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