boards: the amd64 target is PC -- a generic x86-64 UEFI PC, not the SER5

Captain Bob, 2026-10-14: "change to PC, not SER5. it's not supposed to be
specific, but PC generic."  boards/ser5 -> boards/pc, TARGET=PC,
lithos-pc.img; the image was never specific to one machine (the generic
UEFI/ACPI path, FABRIC-3 III).  The Beelink SER5 stays named where the text
means the unit on the bench (its Secure Boot, BIOS, serial header) and in
the records.  Built: lithos-pc.img, system=v4, with its SBOM.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

boards/pc/README.md

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+# PC boot media (generic x86-64, UEFI)
+
+`make boot_image TARGET=PC` builds `build/boards/pc/lithos-pc.img`: a GPT disk with one FAT32
+EFI System Partition, for any x86-64 PC whose UEFI firmware boots a removable medium. Write it to
+a USB stick with `dd`. The partition holds:
+
+- `EFI/BOOT/BOOTX64.EFI`: the amd64 monolithic loader, with the whole kernel embedded. Firmware
+  finds it through the UEFI removable-media path, so no boot entry has to be created.
+- `startup.nsh`: runs the loader if the firmware drops into the UEFI shell.
+- `starforth.cfg`: the boot arguments -- `system=v4` as built; change the word to `system=v3` to
+  boot v3 (and `KERNEL_ARGS="..."` adds to it).
+
+Beside the image, `lithos-pc.spdx` / `.spdx.json`: its SBOM, every file on it with its SHA-256.
+
+Nothing in the image is specific to one machine: it follows the generic UEFI/ACPI path in
+FABRIC-3.md §III. Until 2026-10-14 this target was named `SER5`, after the first PC to be brought
+up -- a Beelink SER5 (AMD Ryzen), which is still the one on the bench (`boards/BRINGUP.md`).
+
+Status: booted in QEMU (q35 + OVMF, from this image as its own disk): v4 as built, v3 with
+`system=v3`. Not yet booted on real hardware.

boards/pc/board.mk

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+# boards/pc/board.mk -- a generic PC: x86-64, UEFI firmware, ACPI.
+#
+# Not one machine: the standards-compliant UEFI path (FABRIC-3.md §III), so
+# any x86-64 PC whose firmware boots a UEFI removable medium. (The first to be
+# brought up on the bench is a Beelink SER5, an AMD Ryzen mini-PC -- see
+# boards/BRINGUP.md.) The monolithic loader embeds the whole kernel, so the
+# image is a GPT disk with one FAT32 EFI System Partition holding
+# EFI/BOOT/BOOTX64.EFI. Write it to a USB stick with dd.
+BOARD_DESC  := Generic PC (x86-64, UEFI, ACPI)
+BOARD_ARCH  := amd64
+BOARD_BOOT  := uefi-esp
+BOARD_IMAGE := lithos-pc.img