refactor(sdk): drop default-quant from model-manager

[Mengsheng Wu (mengshengwu)]

Summary

• Rust core: remove DEFAULT_QUANT untagged-GGUF sentinel, pick_default_quant auto-select, and Error::NoDownloadedQuant. Untagged GGUFs are silently dropped during manifest inference; Store::get_paths(bare-name) keeps a cache-side priority fallback (documented in-code as not a hub-side default).
• FFI + header: drop is_default from geniex_QuantCandidate. geniex_model_query now returns a priority-sorted list; callers grab the head.
• Bindings (Go / Python / Kotlin / JNI): drop is_default field and its plumbing. Python ensure_cached picks candidates[0].
• CLI choosePrecision pre-fills from candidates[0] (SDK-sorted head) instead of the removed flag; no more (default) label.
• Housekeeping in touched files: fold paths.rs into store.rs, extract file_info helper in manifest_builder.rs, drop dead DownloadInfo / VoidPtr / unused log-level wrappers, hide resume / validation mods.

Test plan

• pull org/repo-GGUF:default for a repo that used to rely on the sentinel (e.g. Qwen/Qwen2-1.5B-Instruct-GGUF) → must fail with QuantNotFound (was: succeeded and pulled the untagged fp16).
• geniex model query Qwen/Qwen2-1.5B-Instruct-GGUF → candidate list must not include a default entry; head is Q4_0.
• geniex model pull Qwen/Qwen2-1.5B-Instruct-GGUF with no :quant → interactive picker pre-selects the head (Q4_0) and shows no (default) label.