Code Coverage and Memory Tracking Improvements for MetaCall Core

MetaCall Core is a polyglot runtime written in C/C++ that enables seamless cross-language function calls. Like any large native codebase, it is susceptible to memory leaks and unsafe memory access patterns that are difficult to detect manually and invisible in standard CI. This project improves memory tracking and code coverage reporting across platforms through six deliverables: (1) a Valgrind memcheck CI pipeline integrated into GitHub Actions — already implemented and merged pre-GSoC; (2) systematic fixes to the ~5,579 memory leaks currently detected by CI, reduced from ~99,578 through pre-GSoC contributions (PRs #706, #723, #726); (3) an improved reflect_memory_tracker.h that records file, line, and call-site information for every allocation instead of just counting them; (4) a custom lightweight instrumentation layer using LD_PRELOAD or weak symbol overriding for environments where Valgrind is unavailable; (5) enhanced CI error visibility that surfaces sanitizer-style output for failing tests instead of silent segfaults; and (6) documentation covering the new tracker API, Valgrind output interpretation, and per-loader suppression file strategy. The end result is a MetaCall codebase where memory issues are caught automatically in CI, located precisely when they occur, and actionable for any contributor — not just those with Valgrind expertise.