fix: Allow -1 as sentinel value for heapUsedBytes in ResourceSnapshot

Problem:
- ResourceSnapshot constructor rejected ALL negative values for heapUsedBytes
- ApplicationResourceMonitor.estimateHeapUsage() returns -1 to indicate "not available"
  (per-application heap tracking requires JVMTI instrumentation)
- This caused constructor to throw IllegalArgumentException when creating snapshots
- CRITICAL bug preventing resource monitoring from working

Solution:
- Modified validation to allow -1 as special sentinel value
- Changed: if (heapUsedBytes < 0) to if (heapUsedBytes < -1)
- Updated error message to clarify -1 is allowed for N/A
- Follows standard monitoring convention where -1 = "not available"

Benefits:
- Fixes crash in ApplicationResourceMonitor.collectMetrics()
- Allows resource monitoring to work without heap tracking instrumentation
- Follows common monitoring conventions (-1 for unavailable metrics)
- Still validates against invalid negative values (< -1)

Location: jplatform-api/src/main/java/org/flossware/jplatform/api/ResourceSnapshot.java:57-60

Closes #126

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

Author: Flossy

jplatform-api/src/main/java/org/flossware/jplatform/api/ResourceSnapshot.java

@@ -54,8 +54,9 @@ public ResourceSnapshot(long timestamp, long cpuTimeNanos, long heapUsedBytes,
         if (cpuTimeNanos < 0) {
             throw new IllegalArgumentException("cpuTimeNanos cannot be negative: " + cpuTimeNanos);
         }
-        if (heapUsedBytes < 0) {
-            throw new IllegalArgumentException("heapUsedBytes cannot be negative: " + heapUsedBytes);
+        // Allow -1 to indicate "not available", but reject other negative values
+        if (heapUsedBytes < -1) {
+            throw new IllegalArgumentException("heapUsedBytes cannot be negative (except -1 for N/A): " + heapUsedBytes);
         }
         if (threadCount < 0) {
             throw new IllegalArgumentException("threadCount cannot be negative: " + threadCount);