Implement incremental and generational GC and document performance roadmap

[alexryndin]

Summary

• replace deferred full-collection GC steps with persistent incremental pause -> propagate -> atomic -> sweep phases;
• implement a PUC-like generational collector with object ages, nursery registries, remembered sets, write barriers, minor/major transitions, weak tables and finalizers;
• fix automatic GC pacing after collectgarbage("restart");
• add regression tests for generational minor collections and restart pacing;
• update README with the completed GC work, measured performance bottlenecks and roadmap P15.31-P15.37.

Changed files

• README.md
• src/lua/vm.zig
• tests/smoke/43_generational_minor.lua
• tests/smoke/44_gc_restart_pace.lua

Verification performed before upload

• Zig 0.16.0 Debug build and unit tests;
• 44/44 differential smoke tests;
• iterative dispatch stress under a 1 MiB host stack;
• 29/29 portable/soft upstream suites;
• direct gc.lua, constructs.lua, and verybig.lua;
• full gengc.lua --testc lane.

The branch contains one source commit relative to master; no staging, workflow, archive, base64, generated timing cache, Zig cache, or build artifacts are included.