Add provide/emit! geography article, update spreads article, fix foundations rendering

- New geography article (provide.sx): four primitives, demos, nested scoping,
  adapter comparison, spec explorer links
- Updated spreads article section VI: provide/emit! is now implemented, not planned
- Fixed foundations.sx: ~docs/code-block → ~docs/code (undefined component
  was causing the page to silently fail to render)
- Added nav entry and defpage route for provide/emit! article

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sx/sx/plans/foundations.sx

@@ -20,7 +20,7 @@
     (p "Every layer is definable in terms of the one below. "
        "No layer can be decomposed without the layer beneath it.")
 
-    (~docs/code-block :code
+    (~docs/code :code
       (str "Layer 0: CEK machine      (expression + environment + continuation)\n"
            "Layer 1: Continuations     (shift / reset \u2014 delimited capture)\n"
            "Layer 2: Algebraic effects (operations + handlers)\n"
@@ -69,7 +69,7 @@
 
     (p "SX already implements CEK. It just doesn't name it:")
 
-    (~docs/code-block :code
+    (~docs/code :code
       (str ";; eval-expr IS the CEK transition function\n"
            ";; C = expr, E = env, K = implicit (call stack / trampoline)\n"
            "(define eval-expr\n"
@@ -105,7 +105,7 @@
     (p "Delimited continuations (Felleisen 1988, Danvy & Filinski 1990) "
        "expose the K register as a first-class value:")
 
-    (~docs/code-block :code
+    (~docs/code :code
       (str ";; reset marks a point in the continuation\n"
            "(reset\n"
            "  (+ 1 (shift k        ;; k = \"the rest up to reset\"\n"
@@ -148,7 +148,7 @@
        "an operation (\"perform this effect\") and a handler (\"here's what that effect means\"). "
        "The handler receives the operation's argument and a continuation to resume the program.")
 
-    (~docs/code-block :code
+    (~docs/code :code
       (str ";; Pseudocode \u2014 algebraic effect style\n"
            "(handle\n"
            "  (fn () (+ 1 (perform :ask \"what number?\")))\n"
@@ -317,7 +317,7 @@
     (p "The deepest primitive is not a single thing. "
        "It's a point in a three-dimensional space:")
 
-    (~docs/code-block :code
+    (~docs/code :code
       (str "depth:     CEK \u2192 continuations \u2192 algebraic effects \u2192 scoped effects\n"
            "topology:  sequential \u2192 concurrent \u2192 distributed\n"
            "linearity: unrestricted \u2192 affine \u2192 linear"))
@@ -378,7 +378,7 @@
     (p "If C, E, and K are all data structures (not host stack frames), "
        "the entire computation state is serializable:")
 
-    (~docs/code-block :code
+    (~docs/code :code
       (str ";; Freeze a computation mid-flight\n"
            "(let ((state (capture-cek)))\n"
            "  (send-to-worker state)  ;; ship to another machine\n"
@@ -454,7 +454,7 @@
 
     (p "Add optional effect annotations to function definitions:")
 
-    (~docs/code-block :code
+    (~docs/code :code
       (str ";; Declare what effects a function uses\n"
            "(define fetch-user :effects [io auth]\n"
            "  (fn (id) ...))\n"
@@ -477,7 +477,7 @@
 
     (p "Refactor eval.sx to expose the CEK registers as data:")
 
-    (~docs/code-block :code
+    (~docs/code :code
       (str ";; The CEK state is a value\n"
            "(define-record CEK\n"
            "  :control  expr       ;; the expression\n"
@@ -514,7 +514,7 @@
 
     (p "Extend the CEK machine to support multiple concurrent computations:")
 
-    (~docs/code-block :code
+    (~docs/code :code
       (str ";; Fork: create two CEK states from one\n"
            "(define fork :effects [concurrency]\n"
            "  (fn (cek)\n"
@@ -539,7 +539,7 @@
 
     (p "Add resource-safety constraints:")
 
-    (~docs/code-block :code
+    (~docs/code :code
       (str ";; Linear scope: must be entered, must complete\n"
            "(define-linear open-file :effects [io linear]\n"
            "  (fn (path)\n"