WIP: restore custom completion and embedding endpoints

[JegernOUTT]

Summary

WIP task-planner implementation for restoring custom completion and embedding endpoint support.

Current included cards:

• T-1: endpoint-style taxonomy and role contract foundations
• T-3: capability-aware CustomProvider runtime

Planned follow-up cards will continue landing on this branch:

• backend caps/default selection
• provider HTTP/API roundtrip
• completion and embedding adapters
• GUI role-separated config/defaults
• mock-server coverage and final compatibility sweep

Verification so far

• cargo test -p refact-core provider_types
• cargo test -p refact-core llm_types
• cargo test -p refact-caps-core provider_config
• cargo test -p refact-caps-core model_records
• cargo test -p refact-providers custom
• cargo test -p refact-providers instance

Notes

Draft/WIP PR. More commits will be pushed as task cards complete.

[JegernOUTT]

🔀 Merged latest main into feature/completion_emb_revival

Pushed merge commit 22409e124 (22042c0c1..22409e124).

main shipped the new Refact UI kit (SettingsSection / SettingItem / Field / kit ModelSelector / Tabs, panel-less glass design, lucide icons) while this branch still used the old Radix Card/Flex/Text panels. The new design wins everywhere; the role-separated custom completion + embedding feature was re-implemented on top of it.

Conflict resolutions

Area	Resolution
engine/global_context.rs	merged atomic imports (AtomicU64 + AtomicUsize)
ModelSelector.tsx	kit base + re-added capability (chat/completion/embedding)
ModelSamplingParams.tsx	kit base + capability axis (embedding hidden, completion ctx)
DefaultModels.tsx (+test)	tabbed SettingsSection + Completion & Embedding role tabs
ProviderForm.tsx	kit base + re-wired RoleSeparatedModelConfig for custom providers
RoleSeparatedModelConfig.tsx	rewritten in Field/Select/SettingsGroup kit (aria-labels preserved)
FormSelect.tsx, ModelSelector/AddCustomModel tests	took main's new-design versions
RichModelSelectItem.*	deleted — superseded by kit ModelSelector
modelProviders.ts (+test)	reverted to main's vendor-detection (dropped feature prefix-first grouping that conflicted with main's authoritative test)
ModelCardPopup.tsx	orphaned Radix <Text> → tokenized markup

Verification

• Engine: cargo check -p refact-lsp → exit 0 (3 pre-existing benign warnings)
• GUI: npm run types ✅ · npm run lint ✅ (0 warnings) · full vitest 2599 passed / 0 failed

Note: the feature's "prefix-first" model grouping (custom/…→custom) was dropped during the merge because it was incompatible with main's openai/claude-…→Anthropic test stub. Display-grouping only; no functional impact.