feat: real transients backed by Janet arrays/tables (interop)

Replace the correctness-only transient aliases with real mutable scratch
collections via host interop:
- transient vector -> a Janet array; conj!/assoc!/pop! mutate in place
- transient map -> a Janet table keyed by canonical key (collection keys still
  compare by value); assoc!/dissoc!/conj! mutate in place
- transient set -> a Janet table; conj!/disj! mutate in place
- persistent! freezes back to a pvec / phm / phs
- count/nth/get/contains? work on transients; transient? predicate added

Building a map/set this way avoids the persistent path's per-step bucket-array
copying (transient map build ~35% faster at 20k here); vectors are comparable
since pvec conj is already ~O(1). The mutating ops return the transient and the
source collection is untouched.

spec/transients-spec (34 cases). conformance 218/218, jpm test green.

README.md

@@ -80,7 +80,8 @@ Jolt targets Clojure semantics but runs on Janet, not the JVM. The notable diver
 - **Concurrency / STM.** Single-threaded. No refs, `dosync`, agents, or `send`; `locking` evaluates its body without real locking. Atoms, volatiles, and delays are supported.
 - **Regex.** Compiled to Janet's PEG engine (Janet has no regex). Supported: capturing groups (`[whole g1 …]`), greedy and lazy quantifiers with backtracking, `(?:…)`, lookahead `(?=…)`/`(?!…)`, alternation, anchors `^ $ \b \B`, character classes, and the `(?i)` flag. Not supported: lookbehind, backreferences (`\1`), and named groups (`(?<name>…)`).
 - **Arrays.** Java-style arrays map onto Janet's native types: `byte-array` is a Janet buffer (contiguous, C-backed); `object-array`/`int-array`/`double-array`/etc. are Janet arrays. `aget`/`aset`/`alength`/`aclone` work over both.
-- **Not implemented.** JVM reflection, `proxy`, and the `clojure.repl`/`clojure.template` namespaces. Transients (`transient`/`conj!`/`persistent!`) work but are correctness-only aliases over the persistent collections (no in-place speedup).
+- **Transients.** `transient`/`conj!`/`assoc!`/`dissoc!`/`disj!`/`pop!`/`persistent!` are real mutable scratch collections backed by Janet's native arrays and tables (vectors → arrays, maps/sets → tables), so building a collection with them avoids the per-step copying of the persistent path (notably for maps/sets). `persistent!` freezes back to a persistent value.
+- **Not implemented.** JVM reflection, `proxy`, and the `clojure.repl`/`clojure.template` namespaces.
 
 Supported and Clojure-compatible: chars as a distinct type, lazy/infinite sequences, transducers, destructuring, multimethods with hierarchies, protocols/records (`deftype`/`defrecord`/`reify`/`extend-protocol`), metadata, namespaces, and the reader (`#()`, `#_`, `#?`, tagged literals, `#"…"`).