reader: feature set #{:jolt :default}, clause-order matching (RFC 0002)

jolt no longer satisfies :clj in reader conditionals. The shortcut was a
measured net liability: :clj branches carry JVM interop and JVM-specific
test expectations jolt fails, and they shadowed :default branches jolt
passes. A/B over the suite: clj,default = 4967 assertions / 4324 pass / 119
errors; jolt,default = 5069 / 4470 / 81 (+146 pass, -38 errors, +8 clean
files). Baselines raised to 4470/86.

Matching is now by CLAUSE order like Clojure — the first clause whose key is
in the feature set wins (#?(:default 5 :clj 6) is 5 everywhere); the old code
scanned for :clj first, then :default, regardless of position.

Foreign clj-targeted libraries are a property of the LOADING CONTEXT, not the
platform: reader-features-set! opts a load into a compatibility set, and the
SCI bootstrap/runtime tests load SCI under ["jolt" "clj" "default"] (its
.cljc selects implementations via :clj with no :jolt branches).
JOLT_FEATURES remains the process-wide override.

RFC 0002 records the decision with the measured data; spec 02-reader S18 is
now normative (clause order, documented feature set, per-context override).
Reader tests updated to the portable set + an opt-in round-trip.

docs/spec/02-reader.md

@@ -127,9 +127,14 @@ checks → UNVERIFIED (rows to add).
   platform satisfies, else nothing. `:default` matches any platform.
   `#?@(…)` splices a sequential form into the surrounding context.
 - Feature keys are implementation-defined; each implementation MUST document
-  its feature set. ⚠ jolt currently satisfies `:clj` (inheriting JVM branches
-  in `.cljc` files) — under review; the portable convention is
-  *own-key + `:default`*.
+  its feature set, and SHOULD follow the portable convention *own dialect key
+  + `:default`*. Matching MUST be by **clause order** — the first clause whose
+  key the platform satisfies wins (`#?(:default 5 :clj 6)` is `5` everywhere)
+  — not by key priority. Implementations SHOULD provide a per-loading-context
+  compatibility override for foreign-dialect libraries. (jolt:
+  `#{:jolt :default}`, opt-in via `reader-features-set!`/`JOLT_FEATURES`;
+  decision + A/B data in RFC 0002 — inheriting `:clj` cost 146 suite
+  assertions and 38 errors.)
 - Reader conditionals MUST be an error outside `.cljc`-style reading unless
   the implementation documents otherwise.