fix(fde): fall back to default GRUB menuentry when grubenv has no saved_entry

[Wilbert957]

Problem

cryptpilot-fde show-reference-value fails on a freshly built image that has
never been booted:

saved_entry not found in GRUB environment

load_kernel_artifacts (cryptpilot-fde/src/disk/grub.rs) hard-requires
saved_entry from the GRUB environment block. A never-booted image (a normal
state for an image produced by cryptpilot-convert) has an empty grubenv
(saved_entry is only written at runtime by savedefault). GRUB's selection
order is next_entry > saved_entry > set default, and such images boot the
default entry (set default="0", i.e. the first menuentry), so the reference
extractor cannot determine the boot entry and errors out.

Fix

When saved_entry is absent, fall back to GRUB's default entry by resolving the
first menuentry id from grub.cfg (matching set default="0"), instead of
failing. No image mutation required.

Testing

cargo fmt --check, cargo build -p cryptpilot-fde and cargo clippy -p cryptpilot-fde
pass. Verified with cryptpilot-fde-host show-reference-value --disk <img> against a
GCP/Ubuntu 24.04 image converted by cryptpilot-convert whose grubenv is empty:
reference values for kernel / initrd / grub / shim / kernel_cmdline are now produced
(previously it aborted).

[shankailun-aliyun]

@Wilbert957 ，您好，您的请求已接收，请耐心等待结果。

[shankailun-aliyun]

@Wilbert957 ，您好，未检测到有镜像需要构建，如需重新检测请评论 /start 。