microvm-rootfs: deterministic ext4 image (fixed UUID/hash-seed + SOURCE_DATE_EPOCH)

[bryan-minimal]

rootfs.img was non-reproducible: mke2fs randomizes the filesystem UUID and the directory hash seed and stamps current time on every inode (15924 differing bytes; first divergence at the ext4 superblock UUID, file offset 0x468).

Fix — pin all three:

• -U <fixed-uuid> (filesystem UUID)
• -E hash_seed=<fixed-uuid> (directory htree hashing)
• SOURCE_DATE_EPOCH=0 (mke2fs honors it for inode/superblock timestamps)

Verified byte-reproducible across two from-scratch builds (repro-check diff: no differences).

🤖 Generated with Claude Code

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Reviewing files that changed from the base of the PR and between 263ba22 and 32d94fa.

📒 Files selected for processing (1)

• packages/microvm-rootfs/build.sh

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch bryan/microvm-rootfs-reproducible

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