test(laravel): audit custom builders and relation callbacks

[shuvroroy]

Adds an assertion audit against Larastan for custom-builder chains and relation callbacks. It confirms supported behavior through type, completion, diagnostic, and runtime assertions and records five concrete inference gaps as L55–L59 instead of changing the shared engine during the audit.

The new fixture has 35 active type assertions and 20 explicit known-gap expectations using the existing corpus SKIP convention. Laravel demos exercise custom-builder forwarding, dotted relations, and callbacks in later argument positions.

Validation: full cargo test (14,831 passed), both required clippy runs, cargo fmt --check, PHP syntax/runtime checks (185 Laravel assertions), and Laravel analysis with exactly the three intentional diagnostics.

This is the base of a follow-up stack implementing L55–L59, one task per PR.

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