fat: mount at the id-path from argv; migrate /volumes/usb -> id-path (S2)

The flip that makes the mount path the volume's content id. fat retires its
hardcoded fat_mounts: it reads its mount path from argv[2] (the volume manager
hands it the id-path, e.g. /volumes/fat-12345678, from the FAT serial), mounts
its volume root there, and installs the /system/configuration + /system/logs FHS
rewrites so /system/logs persistence stays decoupled from which volume backs it.
The rewrites are unconditional this increment (the single volume IS the boot
volume); S3 makes them content-conditional across N volumes. Every /volumes/usb
reference migrates to /volumes/fat-12345678 in one commit — the fat-test,
badge-scope-test, and vfs-test fixtures and the four QEMU regexes — plus a new
volume-identity-name case asserting the id-path mount and the /system/logs
rewrite. Discrimination: the regexes now require /volumes/fat-12345678, which the
old hardcoded fat never emitted (it mounted /volumes/usb). Full suite 128/128.

test/qemu_test.py

@@ -179,7 +179,7 @@ CASES = [
      "smp":    4,
      "timeout": 150,
      "qemu_extra": ["-device", "intel-iommu,intremap=off"],
-     "expect": r"(?s)(?=.*/system/kernel: iommu online)(?=.*fat: mounted /volumes/usb)(?=.*fat-test: ok)",
+     "expect": r"(?s)(?=.*/system/kernel: iommu online)(?=.*fat: mounted /volumes/fat-12345678)(?=.*fat-test: ok)",
      "fail":   r"DANOS-TEST-RESULT: FAIL|DANOS-IOMMU-FAULT"},
     # DMA + MSI under translation: interrupt-IN reports arrive through translated DMA and
     # the xHC's MSI/MSI-X still delivers (the 0xFEE00000 interrupt window bypasses second-
@@ -215,7 +215,7 @@ CASES = [
      "smp":    4,
      "timeout": 150,
      "qemu_extra": ["-device", "amd-iommu,dma-remap=on,intremap=off"],
-     "expect": r"(?s)(?=.*iommu online \(AMD-Vi\))(?=.*fat: mounted /volumes/usb)(?=.*fat-test: ok)",
+     "expect": r"(?s)(?=.*iommu online \(AMD-Vi\))(?=.*fat: mounted /volumes/fat-12345678)(?=.*fat-test: ok)",
      "fail":   r"DANOS-TEST-RESULT: FAIL|DANOS-IOMMU-FAULT"},
     # Port I/O grants: a claimed device's io_port resource lets a driver read/write its
     # ports (PS/2 status 0x64), gated by the claim; out-of-range/unclaimed is refused.
@@ -749,13 +749,26 @@ CASES = [
      "expect": r"usb-storage: ready[\s\S]*usb-storage: block 0 signature 0x55aa",
      "fail":   r"DANOS-TEST-RESULT: FAIL"},
     # FAT mount end to end: the fat server mounts the boot usb-storage device (the
-    # FAT32 image) into the VFS at /volumes/usb. A fat-test client then lists and reads
+    # FAT32 image) into the VFS at /volumes/fat-12345678. A fat-test client then lists and reads
     # through the mount — proof of the whole stack: block device -> FAT parse ->
     # VFS routing -> file read.
     {"name": "fat-mount",
      "smp":    4,
      "timeout": 150,
-     "expect": r"fat: mounted /volumes/usb[\s\S]*fat-test: ok",
+     "expect": r"fat: mounted /volumes/fat-12345678[\s\S]*fat-test: ok",
+     "fail":   r"DANOS-TEST-RESULT: FAIL"},
+    # The id-path naming (S2, storage-stack-plan.md). The boot volume mounts at
+    # its CONTENT-derived id-path (/volumes/fat-12345678, from the FAT32 serial
+    # 0x12345678) — never a port name — and keeps its FHS rewrites so /system/logs
+    # persistence still rides the volume. Discrimination: before S2's flip fat
+    # hardcoded /volumes/usb, so the id-path mount line never appears. (Making the
+    # rewrites content-conditional on which volume carries the system is S3.)
+    {"name": "volume-identity-name",
+     "build_case": "fat-mount",
+     "smp":    4,
+     "timeout": 150,
+     "expect": r"(?s)fat: mounted /volumes/fat-12345678"
+               r"[\s\S]*fat: mounted /system/logs",
      "fail":   r"DANOS-TEST-RESULT: FAIL"},
     # The removal lifecycle (V4, docs/volume-manager-plan.md): pull the boot stick
     # mid-run. device_del the usb-storage device -> the bus reports the port empty
@@ -780,7 +793,7 @@ CASES = [
      "qmp_sequence": [
          {"delay": 8, "command": "device_del", "arguments": {"id": "bootstorage"}},
      ],
-     "expect": r"(?s)fat: mounted /volumes/usb"
+     "expect": r"(?s)fat: mounted /volumes/fat-12345678"
                r"[\s\S]*volume-manager: storage for volume \d+ removed; unmounting",
      "fail":   r"DANOS-TEST-RESULT: FAIL"},
     # Volume-manager discovery + probe (V3a, docs/volume-manager-plan.md). Reuses