Give Consomme explicit static IPv4 and gateway identities

Tracking issue: MicroVM support and private snapshot restore.

Motivation

A guest restored into a sandbox network needs predictable guest and gateway
addresses. Static configuration should identify those endpoints exactly rather
than allowing packet handling or address derivation to substitute another identity.
This is also a prerequisite for reasoning about per-attachment network policy.

Proposed Scope

• Represent and validate exact static guest IPv4 and gateway identities in
  Consomme configuration.
• Apply those identities consistently to the relevant address-resolution and
  packet-processing paths.
• Define handling for traffic that does not match the configured identity and
  reject invalid or conflicting configurations with actionable errors.
• Preserve behavior for callers that do not request the new static configuration.
• Keep configuration, enforcement tests, and documentation in the same PR.

Acceptance Criteria

• Deterministic packet tests observe the exact configured guest and gateway
  identities rather than implicitly derived substitutes.
• Traffic with a mismatched identity follows the documented rejection policy.
• Invalid and conflicting configurations fail before the attachment is active.
• Existing default-configuration tests continue to pass.
• Tests cover more than one valid address configuration without depending on
  an external network service.

Dependencies

• F03: Deterministic Consomme TCP validation
  provides the packet-test baseline; this is a testing prerequisite.

The separate egress-policy API is not a prerequisite for static addressing.

Non-Goals

This issue does not introduce flow quotas, snapshot persistence, or runtime egress
policy attachment.

Read 00-microvm.md and the F03 deterministic Consomme TCP validation issue first, then locate Consomme's configuration, address-resolution, and packet-processing paths. Done means deterministic tests cover multiple valid static address configurations, mismatches follow the documented rejection policy, invalid configurations fail before activation, default behavior remains passing, and documentation is included.