Define a canonical, bound IPv4 egress policy

Tracking issue: MicroVM support and private snapshot restore.

Motivation

A sandbox network attachment needs an explicit description of permitted IPv4
egress. Configuration should have one validated representation and an unambiguous
binding to the network identity it governs, so creation and restore cannot apply
different interpretations of the same policy.

Proposed Scope

• Introduce a reusable IPv4 egress-policy resource and validation API.
• Define accepted inputs, canonical representation, and the identity to which a
  validated policy is bound.
• Reject malformed, unsupported, or ambiguous input before a consumer uses it to
  authorize an attachment.
• Define how policy resources are compared and transported without losing their
  binding or validation requirements.
• Keep this PR focused on the policy contract, independent of a specific network
  device or snapshot transaction.

Acceptance Criteria

• Equivalent supported inputs produce equivalent canonical policies.
• Tests cover address and prefix boundaries, invalid inputs, and the defined
  behavior for empty or conflicting rules.
• A policy cannot silently be reused for an incompatible network binding.
• Resource round trips preserve the policy and its binding, with validation
  enforced at the appropriate input boundary.
• Documentation states the allow/deny semantics and unsupported policy forms
  without implying that defining a resource alone enforces it at runtime.

Dependencies

No feature prerequisites. Network attachment reconstruction will consume this
contract separately.

Non-Goals

This issue does not add IPv6 policy, a general-purpose firewall, Consomme flow
limits, or network snapshot persistence.

Start with 00-microvm.md and the issue's proposed scope to establish how the IPv4 egress-policy resource fits the microVM and private snapshot work. Define the accepted inputs, canonical form, network binding, validation boundary, comparison and round-trip behavior, then use the acceptance criteria to verify boundary, invalid-input, conflict, and documentation coverage.