feat(config): add --config YAML configuration system with Hydra compose API

[coketaste]

Summary

Adds --config as a composable, Hydra-based YAML alternative to --additional-context JSON strings, available on both run and build commands.

• New src/madengine/config/ module — HydraConfigLoader (Compose API), ConfigTranslator (YAML → internal context key mapping), ConfigValidator (cross-field checks), with a shared
  deep_merge utility
• Hydra config groups under src/madengine/configs/ — default groups (platform, scheduler, hardware, launcher) and append-only groups (+profile, +env, +tools, +data, +build) with
  pre-built YAML fragments for all supported launchers, hardware profiles, and profiling tools
• Mutual exclusion — --config cannot be combined with --additional-context / --additional-context-file; validated at CLI entry point on both run and build
• Empty-value filtering — empty lists and dicts are excluded from the translated context so they don't shadow existing defaults
• examples/configs/ — three comprehensive annotated templates (local.yaml, slurm.yaml, k8s.yaml) covering every supported field, plus 25 ready-to-run demo files organised by target
  (demo/local/, demo/slurm/, demo/k8s/)
• Fixture fix — dummy_torchrun and dummy_torchrun_multi n_gpus corrected from "1" to "4" so local multi-GPU demo runs show accurate 1N×4G topology

Behaviour

• Config group overrides
  madengine run --tags model --config scheduler=slurm --config launcher=torchrun

• Append-only groups
  madengine run --tags model --config +profile=mi300x_8gpu --config +env=nccl_debug

• User YAML file (highest priority after inline overrides)
  madengine run --config my_job.yaml

• Mix file + inline overrides
  madengine run --config my_job.yaml --config distributed.nnodes=8

▎ When a user YAML sets a distributed launcher, distributed.enabled: true must also be set explicitly — the default launcher: none group sets enabled: false and setting a launcher key
▎ alone does not override it.

Test plan

• pytest tests/unit/test_config_loader.py tests/unit/test_config_schema.py tests/unit/test_config_translator.py tests/unit/test_config_integration.py
  tests/unit/test_hydra_config_loader.py
• madengine run --config examples/configs/demo/local/single-gpu.yaml completes and writes perf.csv
• madengine run --config examples/configs/demo/local/multi-gpu-torchrun.yaml shows 1N×4G topology and torchrun in the launcher column
• madengine run --config my_job.yaml --additional-context '{}' exits with INVALID_ARGS (exit code 4)
• pre-commit run --all-files passes

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