Checked narrow casts; fix runtime require in self-contained-built binaries

byte/short/int/long/char silently wrapped or passed out-of-range values
through; the JVM range-checks (RT.byteCast family). One checked-cast
helper now carries the ranges: a double range-checks ITSELF before
truncating ((byte 1.1) is 1, (byte 127.000001) throws), NaN casts to 0,
ratios and bigdecs truncate, a non-number is CCE, and the throw carries
the JVM message. float range-checks against Float/MAX_VALUE. The
unchecked-* casts now genuinely wrap and sign-fold ((unchecked-byte 200)
is -56 — the old bit-and lost the sign) with doubles saturating like
Java's conversions; unchecked-long/int are host natives. double/float of
a bigdec convert instead of crashing. The no-single-float residue stays
accepted (SPEC.md).

Also fixes #290: a binary built by the SELF-CONTAINED joltc died with
'variable var-deref is not bound' when a namespace loaded at runtime.
The in-process build compiled flat.ss against a clean copy-environment,
which orphans every top-level define in locations the binary's runtime
eval can't see. It now compiles against the default interaction
environment (defines land in the real symbol cells, same as the legacy
fresh-Chez path) and a generated prologue pre-binds each kernel name the
runtime redefines to its kernel value, so the earliest boot reads match
the legacy path's primitive references. requiring-resolve is implemented
(the issue's dynamic-require pattern), and the release workflow smokes a
runtime require in a built binary.

Cast namespaces byte/short/int/long/char now fully clean; cts baseline
5805 -> 5857 pass, 67 baselined namespaces. 7 JVM-certified corpus rows.

host/chez/java/bigdec.ss

@@ -355,6 +355,19 @@
         (else (jolt-num-cast-throw x))))
 (def-var! "clojure.core" "rationalize" jolt-rationalize)
 
+;; double/float of a bigdec is its flonum value.
+(set! jolt-double-slow
+  (let ((prev jolt-double-slow))
+    (lambda (x) (if (jbigdec? x) (jbigdec->flonum x) (prev x)))))
+
+;; narrow casts truncate a bigdec like Number.longValue.
+(set! jolt-cast-truncate-slow
+  (let ((prev jolt-cast-truncate-slow))
+    (lambda (x)
+      (if (jbigdec? x)
+          (truncate (/ (jbigdec-unscaled x) (expt 10 (jbigdec-scale x))))
+          (prev x)))))
+
 ;; compare: add a bigdec arm (enables compare / sort / sorted collections). A
 ;; bigdec vs a plain number compares by value; bigdec vs bigdec is scale-independent.
 (define jbd-prev-compare jolt-compare)

host/chez/java/natives-array.ss

@@ -85,10 +85,8 @@
 (define (na-bytes x) (if (and (jolt-array? x) (eq? (jolt-array-kind x) 'byte)) x (na-byte-array x)))
 (define (na-bytes? x) (and (jolt-array? x) (eq? (jolt-array-kind x) 'byte)))
 (define (na-identity x) x)
-(define (na-byte x)
-  (let ((b (bitwise-and (exact (floor x)) #xff))) (if (>= b 128) (- b 256) b)))
-(define (na-short x)
-  (let ((s (bitwise-and (exact (floor x)) #xffff))) (if (>= s #x8000) (- s #x10000) s)))
+(define (na-byte x) (jolt-byte-cast x))
+(define (na-short x) (jolt-short-cast x))
 
 ;; --- chunked seqs -----------------------------------------------------------
 ;; The chunked-seq accessors (chunked-seq? / chunk-first / chunk-rest / chunk-next)