fix(graph): deduplicate shared mmap weight files across component graphs

[Moeez34]

Problem

When loading models composed of multiple component graphs (e.g. lm_encoder, lm_encoder_text_chunk, lm_encoder_step for models like gemma-4-e2b-it), shared weight files like embed_tokens_per_layer.weights are bound by multiple components.

Previously, weight_cache_ was isolated to each individual CactusGraph instance. As a result, each component graph created its own MappedFile object and issued an independent mmap() system call for shared weights.

On operating systems with per-process virtual address space / mapped-memory ceilings (such as iOS's ~6.5 GiB limit), repeatedly mapping a 1.13 GiB weight file 3+ times alongside other weights exhausts the process address space budget. This caused mmap to fail with ENOMEM ("Cannot map file") during cactus_init.

Solution

1. Process-Wide Thread-Safe Mmap Cache (MappedFileRegistry):

   • Introduced GraphFile::MappedFileRegistry in cactus_graph.h / io.cpp.
   • Maintains a thread-safe registry mapping canonical absolute file paths (std::filesystem::canonical) to std::weak_ptr<GraphFile::MappedFile>.
   • If a weight file is already memory-mapped by any component graph in the process, subsequent requests share the active MappedFile instance instead of creating duplicate virtual memory mappings.

2. Refactored CactusGraph Storage:

   • Updated CactusGraph::mapped_files_ from std::vector<std::unique_ptr<MappedFile>> to std::vector<std::shared_ptr<MappedFile>>.
   • Updated mmap_embeddings, mmap_weights, bind_mmap_weights, and embedding to retrieve shared MappedFile pointers via MappedFileRegistry::get_or_load().

3. Automatic Cleanup (RAII):

   • When all component graphs referencing a weight file are destroyed or reset, the std::shared_ptr reference count reaches zero, triggering MappedFile::~MappedFile() (munmap), cleanly unmapping the region.

Verification

• Added test_shared_mmap_weights_dedup() unit test in cactus-graph/tests/test_io.cpp.
• Verified that multiple CactusGraph instances binding the same weight file receive identical mmap buffer pointers (ptr1 == ptr2) and release the mapping safely upon graph teardown.