feat: add Random, Time, and File effects to C backend

New effects with evidence passing support:

Random effect:
- int(min, max) - random integer in range
- float() - random float 0-1
- bool() - random boolean

Time effect:
- now() - milliseconds since epoch
- sleep(ms) - pause execution

File effect:
- read(path) - read file contents
- write(path, content) - write file
- append(path, content) - append to file
- exists(path) - check if file exists
- delete(path) - delete file
- isDir(path) - check if directory
- mkdir(path) - create directory

Also fixed:
- Function calls as statements now properly emit in generated C
- Return type inference for all effect operations

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

docs/C_BACKEND.md

@@ -275,7 +275,7 @@ See [docs/EVIDENCE_PASSING.md](EVIDENCE_PASSING.md) for details.
 
 ## Future Roadmap
 
-### Phase 2: Perceus Reference Counting
+### Phase 4: Perceus Reference Counting
 
 **Goal:** Deterministic memory management without GC pauses.
 
@@ -292,15 +292,22 @@ fn increment(xs: List<Int>): List<Int> =
 
 If `xs` has refcount=1, the list can be mutated in-place instead of copied.
 
-### Phase 3: More Effects
+### Phase 2: More Effects ✅ COMPLETE
 
-Implement C versions of:
-- `File` (read, write, exists)
-- `Http` (get, post)
-- `Random` (int, bool)
-- `Time` (now, sleep)
+Implemented C versions of:
+- `Random` (int, float, bool) - LCG random number generator
+- `Time` (now, sleep) - using clock_gettime/nanosleep
+- `File` (read, write, append, exists, delete, isDir, mkdir)
 
-### Phase 4: JavaScript Backend
+All effects use evidence passing for handler customization.
+
+### Phase 3: Http Effect
+
+Implement HTTP client:
+- `Http.get(url)` - GET request
+- `Http.post(url, body)` - POST request
+
+### Phase 5: JavaScript Backend
 
 Compile Lux to JavaScript for browser/Node.js:
 - Effects → Direct DOM/API calls