Step 5 piece 6: migrate 23 .sx files to define-library/import

Wraps all core .sx files in R7RS define-library with explicit export
lists, plus (import ...) at end for backward-compatible global re-export.

Libraries registered:
  (sx bytecode)      — 83 opcode constants
  (sx render)        — 15 tag registries + render helpers
  (sx signals)       — 23 reactive signal primitives
  (sx r7rs)          — 21 R7RS aliases
  (sx compiler)      — 42 compiler functions
  (sx vm)            — 32 VM functions
  (sx freeze)        — 9 freeze/thaw functions
  (sx content)       — 6 content store functions
  (sx callcc)        — 1 call/cc wrapper
  (sx highlight)     — 13 syntax highlighting functions
  (sx stdlib)        — 47 stdlib functions
  (sx swap)          — 13 swap algebra functions
  (sx render-trace)  — 8 render trace functions
  (sx harness)       — 21 test harness functions
  (sx canonical)     — 12 canonical serialization functions
  (web adapter-html) — 13 HTML renderer functions
  (web adapter-sx)   — 13 SX wire format functions
  (web engine)       — 33 hypermedia engine functions
  (web request-handler) — 4 request handling functions
  (web page-helpers) — 12 page helper functions
  (web router)       — 36 routing functions
  (web deps)         — 19 dependency analysis functions
  (web orchestration) — 59 page orchestration functions

Key changes:
- define-library now inherits parent env (env-extend env instead of
  env-extend make-env) so library bodies can access platform primitives
- sx_server.ml: added resolve_library_path + load_library_file for
  import resolution (maps library specs to file paths)
- cek_run_with_io: handles "import" locally instead of sending to
  Python bridge

2608/2608 tests passing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/stdlib.sx

@@ -13,6 +13,58 @@
 ;; Replacing them with SX lambdas changes behavior inside shift/reset
 ;; because the transpiled evaluator code uses them directly.
 
+
+(define-library (sx stdlib)
+  (export
+    eq?
+    eqv?
+    equal?
+    boolean?
+    number?
+    string?
+    list?
+    dict?
+    continuation?
+    zero?
+    odd?
+    even?
+    empty?
+    abs
+    ceil
+    round
+    min
+    max
+    clamp
+    first
+    last
+    rest
+    nth
+    cons
+    append
+    reverse
+    flatten
+    range
+    chunk-every
+    zip-pairs
+    vals
+    has-key?
+    assoc
+    dissoc
+    into
+    upcase
+    downcase
+    string-length
+    substring
+    string-contains?
+    starts-with?
+    ends-with?
+    contains?
+    pluralize
+    escape
+    parse-datetime
+    assert)
+  (begin
+
 (define eq?    (fn (a b) (= a b)))
 (define eqv?   (fn (a b) (= a b)))
 (define equal? (fn (a b) (= a b)))
@@ -273,3 +325,9 @@
     (when (not condition)
       (error (or message "Assertion failed")))
     true))
+
+
+)) ;; end define-library
+
+;; Re-export to global namespace for backward compatibility
+(import (sx stdlib))