feat: complete evidence threading in C backend

C backend now fully threads evidence through effectful function calls:

- Track effectful functions via effectful_functions HashSet
- Add has_evidence flag to track context during code generation
- Add LuxEvidence* ev parameter to effectful function signatures
- Transform effect operations to use ev->console->print() when evidence available
- Update function calls to pass evidence (ev or &default_evidence)
- Update main entry point to pass &default_evidence

Generated code now uses zero-cost evidence passing:
  void greet_lux(LuxEvidence* ev) {
      ev->console->print(ev->console->env, "Hello!");
  }

This completes the evidence passing implementation for both interpreter
(O(1) HashMap lookup) and C backend (direct function pointer calls).

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

docs/C_BACKEND.md

@@ -251,13 +251,23 @@ Koka also compiles to C with algebraic effects. Key differences:
 
 ## Current Progress
 
-### Evidence Passing (Zero-Cost Effects) - PARTIALLY COMPLETE
+### Evidence Passing (Zero-Cost Effects) ✅ 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:** ✅ Complete - Full evidence threading through function calls.
 
-**C Backend Threading:** 🔜 Planned - Passing evidence through function calls.
+**Generated code example:**
+```c
+void greet_lux(LuxEvidence* ev) {
+    ev->console->print(ev->console->env, "Hello!");
+}
+
+int main(int argc, char** argv) {
+    greet_lux(&default_evidence);
+    return 0;
+}
+```
 
 See [docs/EVIDENCE_PASSING.md](EVIDENCE_PASSING.md) for details.
 
@@ -265,28 +275,6 @@ See [docs/EVIDENCE_PASSING.md](EVIDENCE_PASSING.md) for details.
 
 ## Future Roadmap
 
-### Phase 1: Complete Evidence Passing in C Backend
-
-**Goal:** Thread evidence through all effectful function calls.
-
-**Current state:** Handler types (`LuxConsoleHandler`, etc.) and `LuxEvidence` struct
-are generated, but not yet used. Effect calls still use hardcoded `lux_console_print()`.
-
-**Required changes:**
-```c
-// Current (hardcoded):
-void greet_lux(LuxString name) {
-    lux_console_print(name);
-}
-
-// 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 (based on Koka benchmarks).
-
 ### Phase 2: Perceus Reference Counting
 
 **Goal:** Deterministic memory management without GC pauses.