build: os-base option to compare the FSDK-built payload against the Flatcar reference

[castrojo]

Design: docs/superpowers/specs/2026-09-13-flatcar-base-migration-design.md (PR #123), supports phase 6. Blocked by #126 and #134.

Reframed by the version-parity decision. The option is no longer "which base ships" - FSDK always ships. It is the harness that makes substitution measurable: build both the FSDK-composed payload and the imported Flatcar reference from one tree, boot both through the same layout, and diff.

Add os-base: fsdk | flatcar-reference:

• fsdk - the shipped payload, composed from FSDK components/* at Flatcar-matched versions.
• flatcar-reference - the imported tree from feat(flatcar): import Flatcar /usr as a boot reference, not as the payload #126, the known-good control.

What to compare

• Boot success and systemd-analyze timings.
• Installed size. The current FSDK DDI is 1.1 GiB compressed; Flatcar's complete /usr is 377 MiB compressed, and packs to 356 MiB as squashfs.
• Sysext attach behavior.
• Per-component behavioral divergence as substitution proceeds. This is the primary output of the experiment, per the design's goals.

Unlike the earlier framing, this option is not removed at cutover. It stays as the regression harness for parity claims.

Acceptance

• just build-ddi honors the option.
• Both variants build on the ghost cluster.
• Both boot through the feat(installer): adopt Flatcar's EFI-SYSTEM/USR-A/USR-B/ROOT partition layout #134 layout.
• Comparison results recorded on this issue and folded into the spec.