feat(registers): ICODE feature-flags decoder (GET NXP SYSTEM INFO)

Add IcodeFeatureFlags (Register) for the 16-bit ICODE feature word
reported by GET NXP SYSTEM INFO (ICODE 3 Table 113; DNA/SLIX2 report a
subset). Read-only capability decoder — decode a captured word to see
what a tag supports. Fields mirror the existing _FEATURE_* constants
one-to-one (asserted in a test). 3 new tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_bitfield.py

@@ -10,6 +10,7 @@ from pm3py.sim import (
     NtagI2CConfig, NtagI2CSession, NtagI2CPtI2C, NT3H2111, NT3H2211,
     Ntag5StatusReg, Ntag5ConfigReg, Ntag5LinkTag,
     NxpKeyPrivileges, IcodeDnaTag,
+    IcodeFeatureFlags,
 )
 
 
@@ -467,3 +468,26 @@ class TestNxpKeyPrivileges:
         text = repr(NxpKeyPrivileges.build(read=True, destroy=True))
         assert "NXP key privileges" in text
         assert "read" in text and "destroy" in text
+
+
+class TestIcodeFeatureFlags:
+    def test_bits_match_module_constants(self):
+        import pm3py.transponders.hf.iso15693.nxp.icode3 as ic3
+        # every declared field must line up with the _FEATURE_* constant of the same name
+        for name, bf in IcodeFeatureFlags.fields():
+            const = getattr(ic3, f"_FEATURE_{name.upper()}")
+            assert (1 << bf.lo) == const, name
+
+    def test_decode(self):
+        # um_pp(0) + counter(1) + originality_sig(8) + high_data_rates(15)
+        w = (1 << 0) | (1 << 1) | (1 << 8) | (1 << 15)
+        f = IcodeFeatureFlags(w)
+        assert f.um_pp is True and f.counter is True
+        assert f.originality_sig is True and f.high_data_rates is True
+        assert f.privacy is False
+        assert int(f) == w
+
+    def test_repr(self):
+        text = repr(IcodeFeatureFlags(0x0001))
+        assert "ICODE feature flags" in text
+        assert "um_pp" in text