feat(models): install and remove weight variants per node

[Lorchie]

A model node that publishes the same weights in several variants (quantizations, precisions) can declare weight_variants next to hf_repo. Each variant is downloaded and deleted on its own from the Extensions drawer, and the params_schema select that picks one labels the variants missing from disk.

Manifest

"params_schema": [
  { "id": "quant", "type": "select", "default": "Q5_K_M",
    "options": [{ "value": "Q4_K_M" }, { "value": "Q5_K_M" }] }
],
"weight_variants": {
  "param": "quant",
  "default": "Q5_K_M",
  "options": [
    { "id": "Q4_K_M", "size_gb": 2.4, "include_prefixes": ["dit/model_Q4_K_M.gguf"], "checks": ["dit/model_Q4_K_M.gguf"] },
    { "id": "Q5_K_M", "include_prefixes": ["dit/model_Q5_K_M.gguf"], "checks": ["dit/model_Q5_K_M.gguf"] }
  ]
}

Validation is mirrored in electron/main/model-sources.ts and api/services/model_sources.py: safe relative paths, no overlap between two variants, download_check outside every variant, no combination with model_sources, and the selecting param must exist (node or extension params_schema) and offer every variant id.

Behaviour

• Every install downloads the shared files first (all variants excluded), then the requested variant — the default one when none is asked for. Files already complete on disk are skipped, so adding a second variant only fetches its own weights.
• New IPC: model:installedWeightVariants (returns null when the install state cannot be read, so the renderer never mistakes it for "none installed") and model:deleteWeightVariant, which unloads the model first and removes only that variant's files, .part included.
• A node counts as installed once its download_check and at least one variant are present.
• Generation refuses a variant that is not installed (GeneratorRegistry.assert_weight_variant_installed, called at the top of _run_generation, so workflows and the agent are both covered). Picking a missing variant in a node opens the extension drawer.

Tests

npm run test:py (115 tests, OK) and npm run test:node (37 + 142, 0 fail) pass; npx eslint . is clean. tsc -b --noEmit reports the 29 errors already present on dev, none in the files touched here.

Not covered: a real Hugging Face download of a variant has not been exercised end to end — the variant files were faked on disk for the UI, deletion and generation-guard runs.