feat: structural-sharing persistent vectors (immutable build) + mutable toggle

Round 2 of the persistent-collections work.

Add a real 32-way branching-trie persistent vector (src/jolt/pv.janet) with a
tail buffer: O(log32 n) conj/assoc/nth/pop, with unchanged subtrees shared by
identity. Vector literals and vec/vector/conj/assoc/subvec/etc. now produce and
maintain these in the default (immutable) build, replacing the old tuple-based
vectors. Every core seq op, the destructurer, IFn application, the printers, =,
and the evaluator's literal/splice paths were taught to handle the pvec type.

Define several Clojure seq fns that were silently leaking to Janet builtins
(some, keep, interleave, flatten, mapcat, interpose) and broke once vectors
became tables; normalize collections through realize-for-iteration everywhere.

Build-time JOLT_MUTABLE flag now selects fast Janet-native mutable collections:
in that mode vectors are arrays (conj appends in place, vector? true, print []),
sharing one representation with lists. Default build is immutable.

Tests: conformance 206/206, features 71/71, jank 119 (baseline). Test helpers
normalized so Janet-level = compares against tuple literals regardless of repr.
The 2 test-load-sci failures (bit-clear/get-method) pre-date this work.

test/clojure-atom-test.janet

@@ -1,6 +1,6 @@
 # Ported from clojure/test_clojure/atoms.clj + systematic atom tests
 (use ../src/jolt/api)
-(defn ct-eval [ctx s] (eval-string ctx s))
+(defn ct-eval [ctx s] (normalize-pvecs (eval-string ctx s)))
 
 (print "Ported Atom Tests")