feat(relations): support conditional polymorphic joins

[first-assist]

Summary

• add opt-in compile-time source and target discriminator conditions for polymorphic relationships over externally managed schemas
• apply fixed, bound-value predicates consistently across single, pageable, DataLoader, fallback, and nested bulk-preload paths
• qualify relation-loader cache identities by source entity so equal field names on different parent types cannot share incompatible results
• require conditional relationships to disable physical foreign-key emission
• release aligned graphql-orm and graphql-orm-macros 0.27.0

Verification

• conditional relationship runtime tests: 2 passed, covering integer, string, optional boolean, float, pageable DataLoader, and argument-free nested bulk-preload paths
• relation definition compile-fail matrix: 4 passed
• affected relation, naming, recursive, schema-IR, semantic-catalogue, and capability fixture suites passed
• SQLite runtime plus PostgreSQL and MSSQL compile lanes passed
• warnings-denied Clippy and Rustdoc passed
• documentation, inventory, release state/policy, workspace dependency, deterministic manifest, SemVer, formatting, and diff checks passed
• the complete default local run reached an unchanged retention trybuild snapshot whose diagnostic wording differs under the installed Rust 1.90 toolchain; no affected source or snapshot is changed, and hosted CI remains the canonical full-suite gate
• committed diff and this PR body contain no consumer-specific vocabulary

Documentation impact

• Documentation updated
• No documentation impact

Explanation: runtime and macros READMEs, changelog, migration guide, relation and macro references, aligned versions, lockfile, and generated workspace inventory document the compatible conditional-relationship contract.