feat: add bcache support

[caniko]

Summary

Adds Linux bcache (block-layer cache) support to disko via three new types:

• bcache_cache — partition content type for cache devices (SSD, eMMC)
• bcache_backing — partition content type for backing devices (HDD, SD card)
• bcache — top-level aggregator (like mdadm) that runs make-bcache, attaches cache to backing, sets cache mode, and delegates to nested content for filesystem creation

Motivation

bcache lets a fast device (SSD/eMMC) act as a transparent block-layer cache for a slower device (HDD/SD card). This is useful for:

• SBCs with eMMC + SD card (e.g., RockPro64) where NixOS store writes are bottlenecked by SD speed
• Servers with SSD cache in front of HDD arrays
• Any setup where adding a cache tier to existing storage is needed without changing filesystems

Design

Follows the established disko multi-device patterns:

• bcache_cache and bcache_backing register devices to $disko_devices_dir (like mdraid/bcachefs backing types)
• bcache aggregator reads from temp dir, creates the bcache set, and manages content (like mdadm/lvm_vg)
• Device discovery uses sysfs backing_dev_name matching for robustness
• Generates boot.bcache.enable = true in NixOS config

Example usage

disko.devices = {
  disk = {
    cache-disk = {
      device = "/dev/sda";
      content.type = "gpt";
      content.partitions.cache = {
        size = "100%";
        content = { type = "bcache_cache"; set = "main"; };
      };
    };
    backing-disk = {
      device = "/dev/sdb";
      content.type = "gpt";
      content.partitions.backing = {
        size = "100%";
        content = { type = "bcache_backing"; set = "main"; };
      };
    };
  };
  bcache.main = {
    type = "bcache";
    device = "/dev/bcache0";
    cacheMode = "writeback";
    content = { type = "filesystem"; format = "ext4"; mountpoint = "/"; };
  };
};

Limitations (can be addressed in follow-up PRs)

• Single-backing device per bcache set (covers ~90% of use cases)
• Device path defaults to /dev/bcache0 — users with multiple bcache sets need to specify explicitly
• Multi-backing would need udev rules for stable device naming

Test plan

• nix flake check --no-build passes (packages, modules, doc generation evaluate cleanly)
• nix build .#tests.bcache — NixOS VM test that verifies bcache device creation, mount, and cache mode
• Real hardware test on RockPro64 (eMMC cache + SD card backing)