docs(roadmap): enrich LLVM backend tasks with proposal links

Add proposal references and implementation details to:
- 21.8 Concurrency Patterns (parallel, spawn, timeout, nursery, channels)
- 21.9 Capabilities & With Pattern (tracking, provision, def impl)
- 21.10 Collections & Iterators (list, map, set, iterator trait)
- 21.13 FFI Support (C FFI, JS FFI, unsafe blocks)
- 21.14 Conditional Compilation (target, config, constants)
- 21.15 Memory Management (ARC, drop, destruction ordering)

Each section now links to relevant approved proposals and includes
more specific implementation tasks with test file locations.

Author: elucidsoft