feat: implement evidence passing for O(1) effect handler lookup

Interpreter changes:
- Add evidence HashMap for O(1) handler lookup instead of O(n) stack search
- Update eval_run to manage evidence when entering/exiting run blocks
- Modify handle_effect to use evidence.get() instead of stack iteration

C backend infrastructure:
- Add handler structs (LuxConsoleHandler, LuxStateHandler, LuxReaderHandler)
- Add LuxEvidence struct containing pointers to all handlers
- Add default handlers that delegate to built-in implementations
- Add Console.readLine built-in implementation

Documentation:
- Create docs/EVIDENCE_PASSING.md explaining design and implementation
- Update docs/C_BACKEND.md with current progress

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/codegen/c_backend.rs

@@ -379,12 +379,78 @@ impl CBackend {
         self.writeln("    return strstr(haystack, needle) != NULL;");
         self.writeln("}");
         self.writeln("");
-        self.writeln("// === Console Effect (built-in) ===");
+        self.writeln("// === Built-in Effect Implementations ===");
         self.writeln("");
         self.writeln("static void lux_console_print(LuxString msg) {");
         self.writeln("    printf(\"%s\\n\", msg);");
         self.writeln("}");
         self.writeln("");
+        self.writeln("static LuxString lux_console_readLine(void) {");
+        self.writeln("    char buffer[4096];");
+        self.writeln("    if (fgets(buffer, sizeof(buffer), stdin)) {");
+        self.writeln("        size_t len = strlen(buffer);");
+        self.writeln("        if (len > 0 && buffer[len-1] == '\\n') buffer[len-1] = '\\0';");
+        self.writeln("        return strdup(buffer);");
+        self.writeln("    }");
+        self.writeln("    return strdup(\"\");");
+        self.writeln("}");
+        self.writeln("");
+        self.writeln("// === Evidence Passing Types ===");
+        self.writeln("// These enable O(1) effect handler lookup instead of O(n) stack search.");
+        self.writeln("// See docs/EVIDENCE_PASSING.md for details.");
+        self.writeln("");
+        self.writeln("// Handler struct for Console effect");
+        self.writeln("typedef struct {");
+        self.writeln("    void (*print)(void* env, LuxString msg);");
+        self.writeln("    LuxString (*readLine)(void* env);");
+        self.writeln("    void* env;");
+        self.writeln("} LuxConsoleHandler;");
+        self.writeln("");
+        self.writeln("// Handler struct for State effect");
+        self.writeln("typedef struct {");
+        self.writeln("    void* (*get)(void* env);");
+        self.writeln("    void (*put)(void* env, void* value);");
+        self.writeln("    void* env;");
+        self.writeln("} LuxStateHandler;");
+        self.writeln("");
+        self.writeln("// Handler struct for Reader effect");
+        self.writeln("typedef struct {");
+        self.writeln("    void* (*ask)(void* env);");
+        self.writeln("    void* env;");
+        self.writeln("} LuxReaderHandler;");
+        self.writeln("");
+        self.writeln("// Evidence struct - passed to effectful functions");
+        self.writeln("// Contains pointers to current handlers for each effect type");
+        self.writeln("typedef struct {");
+        self.writeln("    LuxConsoleHandler* console;");
+        self.writeln("    LuxStateHandler* state;");
+        self.writeln("    LuxReaderHandler* reader;");
+        self.writeln("} LuxEvidence;");
+        self.writeln("");
+        self.writeln("// Default Console handler using built-in implementations");
+        self.writeln("static void default_console_print(void* env, LuxString msg) {");
+        self.writeln("    (void)env;");
+        self.writeln("    lux_console_print(msg);");
+        self.writeln("}");
+        self.writeln("");
+        self.writeln("static LuxString default_console_readLine(void* env) {");
+        self.writeln("    (void)env;");
+        self.writeln("    return lux_console_readLine();");
+        self.writeln("}");
+        self.writeln("");
+        self.writeln("static LuxConsoleHandler default_console_handler = {");
+        self.writeln("    .print = default_console_print,");
+        self.writeln("    .readLine = default_console_readLine,");
+        self.writeln("    .env = NULL");
+        self.writeln("};");
+        self.writeln("");
+        self.writeln("// Default evidence with built-in handlers");
+        self.writeln("static LuxEvidence default_evidence = {");
+        self.writeln("    .console = &default_console_handler,");
+        self.writeln("    .state = NULL,");
+        self.writeln("    .reader = NULL");
+        self.writeln("};");
+        self.writeln("");
         self.writeln("// === List Types ===");
         self.writeln("");
         self.writeln("typedef struct {");