Summary

EPYC server BIOSes (Naples through Turin) currently come back from PSPTool.from_file() with zen_generation = None on every directory and entry — silently. Neither of the two existing detection paths (combo_dir, tertiary self-tag) fires on a single-FET → single $PSP + $BHD layout, and no warning is emitted, so downstream tooling that bins or groups by Zen generation just drops these rows.

This adds a third detection path that runs after FET parsing: scan the PSP_FW_BOOT_LOADER (entry type 0x01) of any ROM whose directories were uniformly left unclassified, read the version major byte, and look it up in a new Directory.BOOTLOADER_VERSION_TO_ZEN map (Naples server through Turin). Apply the result to all directories in that ROM, including the $BHD/$BL2 BIOS directories that don't carry a boot loader of their own.

The back-fill is deliberately conservative:

• Only fires when every directory in the ROM is currently None. In a multi-generation combo BIOS the boot loader belongs to one of several $PSP directories and cannot be safely associated with the sibling $BHD directories — combo_dir / tertiary remain the source of truth for those.
• Emits a warning when a boot loader exists but its version major isn't in the table (replaces one silent-failure mode with a visible one).

Implementation note

The major byte lives at version[1], not version[2]. HeaderFile.version is reverse-sliced from header[0x63:0x5f:-1], so for a Rome boot loader printed as 0.C.0.86 we have v[0]=0x00, v[1]=0x0C, v[2]=0x00, v[3]=0x86. v[2] would silently always read 0x00 and never match.

What's in the PR

A single commit: back-fill code in psptool/directory.py + psptool/psptool.py, a small refactor in psptool/blob.py to expose POSSIBLE_FET_OFFSETS as a class constant (so the synthetic builder can derive its offsets instead of hardcoding them), a synthetic-ROM builder under tests/unit/, and a unit test that exercises the full mapping table without needing fixtures.

Test plan

• Unit tests (no submodule needed): python -m unittest tests.unit.test_zen_generation_backfill -v — 3 tests pass. Covers known majors per BOOTLOADER_VERSION_TO_ZEN, the unrecognized-major warning path, and the no-overwrite-of-pre-classified guarantee.
• Manual end-to-end against the real corpus (locally, before the fixture additions): Naples server (Supermicro H11DSI8, Gigabyte MZ31-AR, Tyan S8026) → Zen 1; Rome → Zen 2; Milan → Zen 3; Genoa → Zen 4; Siena → Zen 4; Turin → Zen 4 + Zen 5 (multi-ROM file). All SHA-256-verified against vendor downloads.

Companion work

The submodule URL cwerling/PSPTool-fixtures is dead and the test corpus needs to move to PSPReverse/Test-PSPTool, plus the new TestZenGenerationBackfill class in tests/integration/test_rom_files.py that exercises this fix on the real corpus. That's deliberately split out as #86 so this PR is self-contained: review the fix here, review the test-infrastructure cleanup separately. Both are individually mergeable in either order.

Out of scope / follow-ups

• Distinguishing Zen 4 from Zen 4c (Bergamo / Siena share Zen 4 boot-loader major byte 0x29 with Genoa).
• Detecting Zen generation for non-PSP ROMs (very old AMD platforms with no PSP). They legitimately stay None.