fix(maven): prefer strong Maven identity for mixed manifests

[mstykow]

Summary

• prefer a Maven primary identity for MANIFEST.MF files only when strong Maven coordinates are provable, while keeping richer OSGi metadata and dependency extraction
• add mixed-manifest unit and parser-golden coverage, including an explicit negative regression for ambiguous manifests that must stay type=osgi
• fix update-parser-golden for extensionless .expected parser fixtures and document the mixed Maven+OSGi behavior in the OSGi improvement docs

Scope and exclusions

• Included:
  • src/parsers/maven/manifest.rs mixed-manifest identity selection
  • targeted parser tests and a new Maven golden fixture for the Jackson-like manifest shape
  • src/golden_maintenance.rs support for formatting extensionless .expected parser outputs
  • docs clarifying mixed-manifest Maven fallback behavior for OSGi parsing
• Explicit exclusions:
  • no broader parser-architecture or assembly-model changes
  • no dual-emission of separate Maven and OSGi package rows from one manifest

Intentional differences from Python

• For mixed Maven+OSGi manifests with strongly provable Maven coordinates, this branch keeps a single emitted package row but prefers a Maven primary identity while preserving OSGi metadata and dependency edges in the same result.

Expected-output fixture changes

• Files changed: testdata/maven-golden/jackson-mixed-manifest/MANIFEST.MF.expected
• Why the new expected output is correct:
  • the fixture models a manifest with both OSGi headers and strong Maven coordinates
  • the parser now emits one Maven package row with java_jar_manifest/pkg:maven/... identity in that case
  • OSGi bundle identity is preserved in extra_data, and OSGi dependency edges from Import-Package / Require-Bundle are still emitted