fix(show-command): model the JSON output as dedicated structs

Replace the `IndexMap<String, PhpMixed>` maps behind `show`'s three output
shapes with structs deriving `serde::Serialize`, so field order, key omission
and PHP's rule that an empty array encodes as `[]` live in the type instead of
being rebuilt at every insertion site.

Typing the fields after PHP's own signatures corrects three divergences:
`description`, `source.url`/`source.reference` and `dist.url`/`dist.reference`
now encode as `null` rather than `""` where the getter returns null, and the
package list follows PHP's `??` chain for the homepage fallback instead of
also skipping empty strings.

Record with TODO(port) that `get_links_for_type` matches composer.json key
names while `show` passes `Link::TYPE_*`, so its link sections never print.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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