[Schema 0.8] Contradictory spec and parser semantics for an omitted network.ingress.hostLoopback

### Relevant area(s)
Linux, macOS, Windows

### Brief description of your issue

The spec and the parser disagree on what an omitted `network.ingress.hostLoopback` means, and the spec contradicts itself.

- **Normative** ([`networking.md:109`](../blob/main/docs/sandbox-policy/0.8.0/networking/networking.md#L109)): "The specific `hostLoopback` value **overrides** `default` for the host-loopback path" — implies absent means inherit `default`.
- **Bubblewrap addendum** (line 481): "An omitted `ingress` section enforces the same deny, since that is the schema's default" — implies absolute deny. (Bubblewrap rejects `allow` for both fields, so both readings coincide there; it never disambiguates.)
- **JSON schema**: both fields optional, no `default` declared.
- **Parser** (`network_parser.rs:300`): `unwrap_or_default()` → `Deny`, independent of `default`.

The readings diverge only when `ingress.default: "allow"` and `hostLoopback` is omitted: inherit → `allow`, parser → `deny`.

Not academic — it changes Windows acceptance. `network_parser.rs:592` requires `hostLoopback: "allow"` for a ProcessContainer proxy without `allowedProxyPeer`; today omission resolves to `deny` and the config is rejected, but under inheritance it would be accepted.

Found while investigating #1021.

### Steps to reproduce
1. Author a 0.8 config with `network.ingress.default: "allow"` and no `hostLoopback`.
2. Observe the parser resolves `host_loopback` to `Deny` instead of inheriting `Allow`.
3. On Windows, add a ProcessContainer `runtimeConfig.networkProxy` without `allowedProxyPeer`; observe rejection with "requires `network.ingress.hostLoopback='allow'`" despite `ingress.default` already being `"allow"`.

### Expected behavior

One documented rule for an omitted `hostLoopback`, applied consistently by the parser and all backends, with the schema stating the default if one exists. Either fix `convert_ingress` to inherit and reword the addendum, or reword the normative "overrides" sentence to state absolute deny.

### Actual behavior

Normative text implies inheritance, an addendum asserts deny, the schema declares nothing, and the parser implements deny. Callers can't tell what omitting the field does.