Fix external launcher startup paths

[zxyasfas]

Summary

• Resolve relative model and MTP paths from the current working directory first, then from the executable directory.
• Resolve Metal source files from the current working directory / env overrides first, then from the executable directory.
• Add GET /health as a lightweight liveness endpoint for process managers and model routers.

Fixes #49

Validation

• git diff --check
• Not run: make test (this Windows environment does not provide the POSIX/macOS headers used by ds4, such as sys/mman.h and arpa/inet.h)