Disable implicit join output row cap

[justinjoy]

Summary

• Disable the physical-RAM-derived join row cap when WIRELOG_JOIN_OUTPUT_LIMIT is unset, empty, or zero.
• Preserve explicit positive decimal caps, clamp them to UINT32_MAX, and reject malformed or overflowing values before acquiring session resources.
• Update memory-policy documentation and add tests for defaults, invalid values, retry, clamp boundaries, worker configuration, and high-water accounting.

This is the implicit row-cap portion of #1369. It does not close #1369; remaining allocation classes are still tracked there and in follow-up work.

Validation

• test_join_limit: 8/8
• join_overflow, memory_admission_join, k_fusion_correctness, tdd_multiworker, join_batch_resume, session, session_admission, session_options, wirelog_public_api, worker_session: passed
• ABI checks: public header surface, public API macro, public API exports passed
• uncrustify --check on changed C files and git diff --check: passed

Binary-size policy remains informational until a stable version is declared.