fix: Final Zig 0.15.2 syntax fixes

- duckdb.zig: Remove enum values (use default)
- png.zig: Mark unused bimodal params with _:
- transport_skins.zig: Mark probe params with _:

Refs: RFC-0015

l0-transport/png.zig

@@ -179,7 +179,7 @@ pub const PngState = struct {
         return scale / std.math.pow(f64, u, 1.0 / shape);
     }
     
-    fn sampleBimodal(self: *Self, mean: f64, stddev: f64) f64 {
+    fn sampleBimodal(self: *Self, _: f64, _: f64) f64 {
         // Two modes: small (600) and large (1440), ratio 1:3
         if (self.nextF64() < 0.25) {
             // Small mode around 600 bytes

l0-transport/transport_skins.zig

@@ -362,16 +362,12 @@ pub const SkinProber = struct {
         });
     }
     
-    fn probeRaw(self: SkinProber, timeout_ms: u32) !bool {
+    fn probeRaw(_: SkinProber, _: u32) !bool {
-        _ = self;
-        _ = timeout_ms;
         // TODO: Implement UDP probe
         return false;
     }
     
-    fn probeHttps(self: SkinProber, timeout_ms: u32) !bool {
+    fn probeHttps(_: SkinProber, _: u32) !bool {
-        _ = self;
-        _ = timeout_ms;
         // TODO: Implement HTTPS probe
         return true; // Assume HTTPS works for now
     }

l4-feed/duckdb.zig

@@ -16,9 +16,9 @@ pub const Result = opaque {};
 pub const Appender = opaque {};
 
 /// State types
-pub const State = enum(u32) {
+pub const State = enum {
-    success = 0,
+    success,
-    error = 1,
+    error,
 };
 
 /// C API Functions