fix: limit extends chain depth to prevent stack overflow DoS

[Jaro-c]

Problem

1. SEC-005: compose/extends.rs resolution is unbounded — a deeply nested extends: chain (or cross-file chain) causes a stack overflow, crashing the process (DoS).
2. EXT-001: Cross-file circular extends: (A.yml:svc extends B.yml:svc extends A.yml:svc) is not detected and causes unbounded recursion.

Fix

Add a depth counter to resolve_one_extends and resolve_one_extends_in_memory. Return Err(ComposeError::Extends(...)) when depth exceeds 16.

The same depth limit naturally caps cross-file circular chains.