define scoutd kernel cmdline boostrap contract

[t0gun]

This PR adds the first kernel command line bootstrap contract for scoutd. It defines a small scoutd owned schema for early guest boot and parses only the values needed before networking and MMDS are available.
The new parser reads the kernel command line, extracts the required bootstrap fields, validates them, and rejects malformed, duplicate, or missing values. On invalid bootstrap input, scoutd now fails fast during early boot instead of continuing with a bad guest configuration.

This keeps the kernel command line intentionally small and focused while setting up the next phase of work, which is guest networking bring up. Build and tests pass with Zig 0.15.2.

[gemini-code-assist]

Code Review

This pull request introduces a kernel command-line parser in src/cmdline.zig to extract network configuration for the scoutd service. It includes logic for reading /proc/cmdline, parsing IPv4 addresses with CIDR prefixes, and handling potential errors during the bootstrap process. Feedback suggests using std.mem.tokenizeAny for more robust tokenization of command-line arguments and std.mem.indexOfScalar for more efficient character searching.