Optimize: TTL cleanup should be on-demand only

[dapi]

Problem

Currently RemoveExpired() is called on EVERY port-selector run (line 176 in main.go), even when there are plenty of free ports available. This is inefficient and unnecessary.

The TTL mechanism should only be used as a last resort when the port range is exhausted.

Current Behavior

Every time a user runs port-selector:

1. All allocations are scanned for expiration
2. Expired allocations are removed (wasting cycles)
3. Then search for free port proceeds

Example: With 1000 ports and only 100 allocated, we still scan all for TTL on every run.

Desired Behavior

Lazy cleanup approach:

1. Try to find a free port in the configured range (normal allocation)
2. If all ports are busy/frozen:
   • Check for expired allocations (last resort)
   • Find the oldest (by LastUsedAt or AssignedAt)
   • Verify it's actually free with port.IsPortFree()
   • Remove and immediately allocate to current directory
3. If after cleanup still no free ports: return error

Benefits

• ✅ Faster normal operation (skip TTL scan)
• ✅ Safer cleanup (only remove when necessary, verify port is free)
• ✅ More predictable behavior
• ✅ Prevent race conditions (remove + allocate atomically within WithStore)

Related Code

• main.go:176 - unconditional RemoveExpired() call
• main.go:195-220 - port finding logic
• allocations.go:439-464 - RemoveExpired() function
• internal/port/checker.go - IsPortFree() function

Implementation Strategy

1. Move RemoveExpired() call from line 176 into the port finding loop
2. Only call when no free port found in range:

   // Find free port (excluding frozen/locked)
   freePort, err := port.FindFreePortWithExclusions(...)
   if err != nil {
       if errors.Is(err, port.ErrAllPortsBusy) {
           // Last resort: try cleanup
           removed := store.RemoveExpired(ttl)
           if removed > 0 {
               // Retry finding free port
               freePort, err = port.FindFreePortWithExclusions(...)
           }
       }
       if err != nil {
           return fmt.Errorf("all ports busy or frozen")
       }
   }

3. Optionally: add GetOldestAllocation() method to prefer oldest allocations for removal
4. Consider: log when cleanup is triggered (warning/info level)

Testing

• Unit test: verify cleanup is NOT called when ports available
• Integration test: verify cleanup IS called when all ports busy
• Integration test: verify removed port can be immediately reallocated