fix: limit schema compilation depth

[mcollina]

Summary

• reject excessively nested schemas before recursive validation and code generation
• apply depth accounting to inline, external, and reference-linked schemas, including validator-only keywords
• detect circular object/reference graphs deterministically and walk validator dependencies iteratively
• expose and document a bounded maxDepth option (default and maximum: 100)

Security

Fixes stack exhaustion during schema compilation reported in GHSA-2CPC-HGRF-32M6.

Validation

• npm test (513 tests, 100% statement/branch/function/line coverage)
• npm run lint
• npm run test:typescript
• reproduced the 718-level advisory PoC and confirmed it returns a controlled depth error instead of RangeError
• exercised the highest accepted depth and first rejected depth across inline/reference-linked and eager/lazy validator paths

[Tony133]

lgtm