feat(audit): 1.3 — CPU collector (dmidecode type 4, microcode)

- cpu.go: collectCPUs(), parseCPUs(), parseCPUSection()
- splitDMISections(): splits multi-section dmidecode output generically
- parseFieldLines(): reusable key→value parser for DMI sections
- parseCPUStatus(): Populated/Unpopulated → OK/WARNING/EMPTY/UNKNOWN
- parseSocketIndex(): CPU0/Processor 1/Socket 2 → integer
- cleanManufacturer(): strips (R), Corporation, Inc. suffixes
- parseMHz(), parseInt(): field value parsers
- Serial fallback: <board_serial>-CPU-<socket> when DMI serial absent
- readMicrocode(): /sys/devices/system/cpu/cpu0/microcode/version
- cpu_test.go: dual-socket, unpopulated skipped, status, socket, manufacturer, MHz

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

audit/internal/collector/testdata/dmidecode_type4.txt

@@ -0,0 +1,66 @@
+# dmidecode 3.5
+Getting SMBIOS data from sysfs.
+SMBIOS 3.1.1 present.
+
+Handle 0x0004, DMI type 4, 48 bytes
+Processor Information
+	Socket Designation: CPU0
+	Type: Central Processor
+	Family: Xeon
+	Manufacturer: Intel(R) Corporation
+	ID: F7 06 08 00 FF FB EB BF
+	Signature: Type 0, Family 6, Model 143, Stepping 7
+	Flags:
+		FPU (Floating-point unit on-chip)
+		VME (Virtual mode extension)
+		DE (Debugging extension)
+	Version: Intel(R) Xeon(R) Gold 6530
+	Voltage: 1.6 V
+	External Clock: 100 MHz
+	Max Speed: 4000 MHz
+	Current Speed: 2100 MHz
+	Status: Populated, Enabled
+	Upgrade: Other
+	L1 Cache Handle: 0x0006
+	L2 Cache Handle: 0x0007
+	L3 Cache Handle: 0x0008
+	Serial Number: Not Specified
+	Asset Tag: Not Specified
+	Part Number: Not Specified
+	Core Count: 32
+	Core Enabled: 32
+	Thread Count: 64
+	Characteristics:
+		64-bit capable
+		Multi-Core
+		Hardware Thread
+
+Handle 0x0005, DMI type 4, 48 bytes
+Processor Information
+	Socket Designation: CPU1
+	Type: Central Processor
+	Family: Xeon
+	Manufacturer: Intel(R) Corporation
+	ID: F7 06 08 00 FF FB EB BF
+	Signature: Type 0, Family 6, Model 143, Stepping 7
+	Flags:
+		FPU (Floating-point unit on-chip)
+	Version: Intel(R) Xeon(R) Gold 6530
+	Voltage: 1.6 V
+	External Clock: 100 MHz
+	Max Speed: 4000 MHz
+	Current Speed: 2100 MHz
+	Status: Populated, Enabled
+	Upgrade: Other
+	L1 Cache Handle: 0x000A
+	L2 Cache Handle: 0x000B
+	L3 Cache Handle: 0x000C
+	Serial Number: Not Specified
+	Asset Tag: Not Specified
+	Part Number: Not Specified
+	Core Count: 32
+	Core Enabled: 32
+	Thread Count: 64
+	Characteristics:
+		64-bit capable
+		Multi-Core

audit/internal/collector/testdata/dmidecode_type4_disabled.txt

@@ -0,0 +1,31 @@
+# dmidecode 3.5
+Getting SMBIOS data from sysfs.
+SMBIOS 3.1.1 present.
+
+Handle 0x0004, DMI type 4, 48 bytes
+Processor Information
+	Socket Designation: CPU0
+	Type: Central Processor
+	Family: Xeon
+	Manufacturer: Intel(R) Corporation
+	Version: Intel(R) Xeon(R) Gold 6530
+	Max Speed: 4000 MHz
+	Current Speed: 2100 MHz
+	Status: Populated, Enabled
+	Serial Number: Not Specified
+	Core Count: 32
+	Thread Count: 64
+
+Handle 0x0005, DMI type 4, 48 bytes
+Processor Information
+	Socket Designation: CPU1
+	Type: Central Processor
+	Family: Unknown
+	Manufacturer: Unknown
+	Version: Unknown
+	Max Speed: Unknown
+	Current Speed: Unknown
+	Status: Unpopulated
+	Serial Number: Not Specified
+	Core Count: N/A
+	Thread Count: N/A