fix(windows): Suppress unused variable warnings in discovery.rs

## Problem

Windows builds were generating 6 warnings in the discovery module:
- 3 x "unused variable: `iter`"
- 3 x "variable does not need to be mutable"

These warnings appeared in all Windows build steps (Build, Run tests, Build release)
due to platform-specific conditional compilation.

## Root Cause

On Windows, the `iter` variable is created but never used because:
1. Unix uses: `iter.next_with_timeout(Duration::from_millis(100))`
2. Windows uses: `Ok(None)` (method doesn't exist on Windows)

The `iter` variable is necessary for Unix but unused on Windows, causing
the compiler to warn about unused mutable variables.

## Solution

Added `#[cfg_attr(windows, allow(unused_variables, unused_mut))]` attributes
to all 3 iterator creation sites:

**Line 203**: ICMP Echo Request iterator
**Line 284**: ICMPv6 Echo Request iterator
**Line 407**: NDP Neighbor Solicitation iterator

This tells the compiler to suppress these warnings only on Windows builds,
where the variables are legitimately unused.

## Testing

**Linux**: ✅ ALL PASS
- cargo build --release: 0 warnings
- cargo clippy: 0 warnings
- cargo test: 270 passed, 5 ignored

**Expected Windows**: ✅ 0 warnings
- All 6 warnings will be suppressed by cfg_attr

## Technical Details

**Approach**: Platform-specific warning suppression
- Unix: Uses `iter` normally, no warnings
- Windows: Suppresses warnings for intentionally unused variable

**Alternative Approaches Considered**:
1. ❌ Remove `iter` on Windows - breaks code structure
2. ❌ Use `_iter` prefix - doesn't fix "unused mut" warning
3. ✅ `cfg_attr` - Clean, explicit, platform-specific

## Impact

- Zero functional changes
- Cleaner Windows build output (0 warnings)
- Better CI experience (no noise in logs)
- Makes actual warnings more visible

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: doublegate

crates/prtip-scanner/src/discovery.rs

@@ -200,6 +200,7 @@ impl DiscoveryEngine {
 
         // Wait for Echo Reply (Type 0) with timeout
         let start = std::time::Instant::now();
+        #[cfg_attr(windows, allow(unused_variables, unused_mut))]
         let mut iter = icmp_packet_iter(&mut rx);
 
         while start.elapsed() < self.timeout {
@@ -280,6 +281,7 @@ impl DiscoveryEngine {
 
         // Wait for Echo Reply (Type 129) with timeout
         let start = std::time::Instant::now();
+        #[cfg_attr(windows, allow(unused_variables, unused_mut))]
         let mut iter = pnet::transport::icmpv6_packet_iter(&mut rx);
 
         while start.elapsed() < self.timeout {
@@ -402,6 +404,7 @@ impl DiscoveryEngine {
 
         // Wait for Neighbor Advertisement (Type 136)
         let start = std::time::Instant::now();
+        #[cfg_attr(windows, allow(unused_variables, unused_mut))]
         let mut iter = pnet::transport::icmpv6_packet_iter(&mut rx);
 
         while start.elapsed() < self.timeout {