rustdoc-json: items may reference non-existent IDs.

To reproduce, get rust-src from 2026-03-05, run cargo rustdoc -p core --output-format json -Z unstable-options -- --document-private-items.

In the generated json file, any reference to ParseFloatError has a non-existent ID.

For example, this use dec2flt::ParseFloatError statement in core/src/num/mod.rs:

{
"7473": {
      "id": 7473,
      "crate_id": 0,
      "name": null,
      "span": {
        "filename": "core/src/num/mod.rs",
        "begin": [
          62,
          1
        ],
        "end": [
          62,
          34
        ]
      },
      "visibility": "public",
      "docs": null,
      "links": {},
      "attrs": [
        {
          "other": "#[attr = CfgTrace([Not(NameValue { name: \"no_fp_fmt_parse\", value: None, span: core/src/num/mod.rs:61:11: 61:26 (#0) }, core/src/num/mod.rs:61:10: 61:27 (#0))])]"
        },
        {
          "other": "#[attr = Stability {stability: Stability {level: Stable {since: Version(RustcVersion { major: 1, minor: 0, patch: 0 })},\nfeature: \"rust1\"}}]"
        }
      ],
      "deprecation": null,
      "inner": {
        "use": {
          "source": "dec2flt::ParseFloatError",
          "name": "ParseFloatError",
          "id": 634,
          "is_glob": false
        }
      }
    }
}

There is no Item or ItemSummary with Id 634.

Reproduce the issue with the listed rust-src revision and cargo rustdoc command, then inspect the generated JSON entry in core/src/num/mod.rs for ParseFloatError and ID 634. Trace how rustdoc JSON resolves referenced items and add a regression check ensuring every referenced ID has a corresponding Item or ItemSummary.