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>

docs/C_BACKEND.md

@@ -249,35 +249,43 @@ Koka also compiles to C with algebraic effects. Key differences:
 
 ---
 
+## Current Progress
+
+### Evidence Passing (Zero-Cost Effects) - PARTIALLY COMPLETE
+
+**Interpreter:** ✅ Complete - O(1) HashMap lookup instead of O(n) stack search.
+
+**C Backend Infrastructure:** ✅ Complete - Handler structs and evidence types generated.
+
+**C Backend Threading:** 🔜 Planned - Passing evidence through function calls.
+
+See [docs/EVIDENCE_PASSING.md](EVIDENCE_PASSING.md) for details.
+
+---
+
 ## Future Roadmap
 
-### Phase 1: Evidence Passing (Zero-Cost Effects)
+### Phase 1: Complete Evidence Passing in C Backend
 
-**Goal:** Eliminate runtime effect handler lookup.
+**Goal:** Thread evidence through all effectful function calls.
 
-**Current approach (slow):**
-```rust
-// O(n) search through handler stack
-for handler in self.handler_stack.iter().rev() {
-    if handler.effect == request.effect {
-        return handler.invoke(request);
-    }
-}
-```
+**Current state:** Handler types (`LuxConsoleHandler`, etc.) and `LuxEvidence` struct
+are generated, but not yet used. Effect calls still use hardcoded `lux_console_print()`.
 
-**Evidence passing (fast):**
+**Required changes:**
 ```c
-typedef struct {
-    Console* console;
-    FileIO* fileio;
-} Evidence;
+// Current (hardcoded):
+void greet_lux(LuxString name) {
+    lux_console_print(name);
+}
 
-void greet(Evidence* ev, const char* name) {
-    ev->console->print(ev, name);  // Direct call, no search
+// Target (evidence passing):
+void greet_lux(LuxEvidence* ev, LuxString name) {
+    ev->console->print(ev->console->env, name);
 }
 ```
 
-**Expected speedup:** 10-20x for effect-heavy code.
+**Expected speedup:** 10-20x for effect-heavy code (based on Koka benchmarks).
 
 ### Phase 2: Perceus Reference Counting