Chez: runtime eval / load-string / defmacro on the zero-Janet spine

The compiler image is already resident at runtime on the Chez spine, so eval
and load-string are just wiring: make them clojure.core functions instead of
analyzer special forms.

- eval / load-string are now functions, not special forms. Dropped "eval" from
  the host-contract special-symbol lists so it resolves as an ordinary var, and
  def-var! both in compile-eval.ss. eval takes an already-read form (e.g. from
  quote/list) and compiles+evals it in the current ns; load-string reads every
  form from a source string and evals each, returning the last.
- Runtime defmacro: jolt-compile-eval-form intercepts a (defmacro ...) form
  before analysis, defs the expander fn + mark-macro!s the var, exactly as
  emit-image.ss does at build time. The two helpers (macro-form? / defmacro->fn)
  move to compile-eval.ss and emit-image.ss reuses them.
- Top-level (do ...) is now unrolled form-by-form, like Clojure, so a defmacro
  or def in an earlier subform is visible (macro flag set / var interned) before
  a later subform is analyzed. This is what makes multi-form -e with a macro work.

Seed is byte-identical (no source references eval), so no re-mint; bootstrap-test
still passes. Zero-Janet corpus 2534 -> 2544 (eval/load-string cases now run),
0 new divergences; floor raised. Prelude corpus, JVM cert, full Janet gate green.

jolt-r8ku

test/chez/run-corpus-zero-janet.janet

@@ -166,7 +166,7 @@
 
 # Regression floor: raise as the Chez-hosted compiler closes gaps. The gate fails
 # on any NEW divergence or if pass drops below the floor. Strided runs scale to 0.
-(def base-floor (scan-number (or (os/getenv "JOLT_CHEZ_ZJ_FLOOR") "2534")))
+(def base-floor (scan-number (or (os/getenv "JOLT_CHEZ_ZJ_FLOOR") "2544")))
 (def floor (if (os/getenv "JOLT_CORPUS_LIMIT") 0 base-floor))
 (when (or (> (length diverged) 0) (< pass floor))
   (printf "REGRESSION: pass %d < floor %d or %d new divergence(s)" pass floor (length diverged)))