Part of #202.

Summary

Add the plumbing for per-distro GRUB fixups in the cidata seed without changing behavior for any current image. All KNOWN_IMAGES entries default to GrubFix::None; flipping them lands in the follow-up.

Changes

src/image.rs:

• Add GrubFix enum:

  #[derive(Clone, Copy, Debug, PartialEq, Eq)]
  pub enum GrubFix {
      None,
      GrubDefaultDropIn,   // /etc/default/grub.d/ is sourced + update-grub
      GrubDefaultInPlace,  // edit /etc/default/grub + grub2-mkconfig
  }

• Add grub_fix: GrubFix field on KnownImage; default all rows to GrubFix::None for now.
• Thread known.grub_fix through ensure_cidata_seed into the SeedSpec it constructs.

src/cloud_init.rs:

• Add structured fields on SeedSpec so bhx-injected entries don't have to ride on extra_user_data:

  pub runcmd: Vec<Vec<String>>,
  pub write_files: Vec<WriteFile>,
  pub grub_fix: GrubFix,

• Extend render_user_data to emit single runcmd: and write_files: blocks built from those structured fields. extra_user_data continues to be appended verbatim — cloud-init's documented list_extend merge for duplicate top-level list keys should concat operator-supplied runcmd: onto bhx's, but verify in Issue C before claiming this works.
• Private helper per GrubFix variant that emits the right runcmd + write_files entries:
  • GrubDefaultDropIn: write /etc/default/grub.d/99-bhx-console.cfg containing GRUB_CMDLINE_LINUX=\"\$GRUB_CMDLINE_LINUX console=hvc0\", GRUB_TIMEOUT_STYLE=menu, GRUB_TIMEOUT=5; runcmd [update-grub]. Use YAML block scalar (|) for the content to sidestep quoting.
  • GrubDefaultInPlace: runcmd seds /etc/default/grub to append console=hvc0 to GRUB_CMDLINE_LINUX, set GRUB_TIMEOUT_STYLE=menu, set GRUB_TIMEOUT=5; then grub2-mkconfig -o /boot/grub2/grub.cfg.

src/profile.rs:

• Only if it constructs SeedSpec literals — default the new fields to empty / GrubFix::None.

Footguns / notes for the implementer

• console=hvc0 must be appended, not prepended: the kernel's /dev/console symlink follows the last console= argument.
• Don't wrap the runcmd in set -e. grub2-mkconfig exits non-zero on benign os-prober failures.
• One-line comment on the runcmd that it runs late enough for /boot to be mounted — anyone tempted to move this to bootcmd "to be earlier" will silently break images with a separate /boot partition.

Test plan (hardware-free)

• Unit test: render SeedSpec for each GrubFix variant; assert generated YAML matches a fixture string.
• Unit test: combine structured runcmd with operator-supplied extra_user_data; parse the resulting YAML and confirm both runcmd: blocks are well-formed.
• cargo fmt --check, cargo clippy --all-targets -- -D warnings, cargo test clean.

Acceptance

• CI gates clean.
• No KNOWN_IMAGES entry changes behavior (all stay GrubFix::None).
• Renderer emits one runcmd: and one write_files: block; YAML round-trips through serde_yaml or equivalent without errors.