model_specs: declare the MiniMax Music3 and H3 options the sessions read

[CryptVenture]

Split out of #372 as requested. This PR declares the MiniMax Music3 and H3 options their sessions already read; the description-only fixes, the VoxCPM1 defaults, the strict contract fixes and the ASR/aligner/codec declarations are separate PRs.

Additive throughout: no existing declaration changes, so nothing that validated before fails now.

The changes

minimax_music3 — five request options and one session option, all read today:

Option	Read in
flow_uncond_interval	src/community_models/minimax_music3/flow_sampler.cpp, session.cpp
flow_uncond_warmup	src/community_models/minimax_music3/flow_sampler.cpp, session.cpp
ensemble_takes	src/community_models/minimax_music3/session.cpp
ensemble_prefix_frames	src/community_models/minimax_music3/pipeline.cpp, session.cpp
flow_chunk_hop_frames	src/community_models/minimax_music3/pipeline.cpp, session.cpp
session minimax_music3.pipeline_overlap	src/community_models/minimax_music3/pipeline.cpp, session.cpp

These arrived with the performance pack and none were declared. ensemble_takes returns several named outputs and had no way to be requested at all.

minimax_h3 — first_block_cache_start_percent and first_block_cache_end_percent (src/community_models/minimax_h3/session.cpp, pipeline.cpp). This is the primary cache window whose _sigma override was already declared, so the spec exposed the fine adjustment while hiding the control it adjusts.

Validation

python3 tools/check_loader_catalog_sync.py
# ok: runtime loaders, model_specs, and model_manager_v2 are in sync

audiocpp_model_manager list --repository-root .        # exit 0
# parses every on-disk spec through the C++ schema loader

Verified by reading the option-parsing code named above. Neither package is installed on this machine, so no end-to-end generation was run.

Scope

Two spec files, +66. The generated WebUI bundle is deliberately excluded — catalog.ts inlines model_specs/*.json at frontend build time and the bundle is not byte-reproducible, so regenerating it in every PR of this split would make the PRs conflict with each other. Happy to send one bundle-regeneration PR once the series lands.

[CryptVenture]

Closing for now to stay inside the 3-concurrent-PR policy (see the discussion on #422). Nothing is wrong with the change and CI is green on it; I will reopen it as review slots free.