fix(provider): inline MTP inspection resolves HF-cache symlinks and rejects loudly

[Gajesh2007]

Problem

Bug 1 from Layr-Labs/mlx-swift-lm#106's Discovered bugs: SpecDecStore.inspectInlineArtifact required regularFileSize — which deliberately rejects symlinks — on the checkpoint directory, config.json, model.safetensors.index.json, and every selected shard. HF-cache snapshots materialize all of those as symlinks into blobs/, so on the layout production checkpoints actually arrive in, inline MTP was rejected with status inline_artifact_invalid and zero log output: the drafter silently never activates, the slot looks healthy, and mtp_enabled simply reads false.

This is not hypothetical: it silently poisoned a paired Qwen3.6 measurement session on 2026-08-13 (MTP-on runs were secretly MTP-off; the session was salvaged with APFS-cloned snapshots). It is also the only reason production dodged the 0.70x serial-verify regression fixed by mlx-swift-lm#106.

Change

1. Symlinks are resolved, not rejected — inline inspection only. Directory, config, index, and shard checks resolve symlinks (URL.resolvingSymlinksInPath) and validate the resolved target (exists, regular file, size bounds). A broken link keeps its unresolvable component, fails the attribute read, and is rejected by name. Operator-provided local overrides (inspectLocalArtifact) still reject symlinks: there the directory itself is untrusted input; the inline directory is the already-verified target checkpoint.
2. Fail-loud — inspectInlineArtifact now returns Result<SpecDecArtifact, InlineArtifactRejection> where every rejection carries the concrete reason and file path, and is also logged at warning level (darkbloom.SpecDecStore). Both inline_artifact_invalid producers (funnel admission and the pre-load revalidateForLoad gate) now surface the reason; no status-only rejection remains.

Behavior

flowchart LR
  subgraph Before
    A1["HF-cache snapshot<br/>(shards = symlinks into blobs/)"] --> B1["regularFileSize() rejects symlink"] --> C1["status inline_artifact_invalid<br/>ZERO log output<br/>MTP silently inert; measurements poisoned"]
  end
  subgraph After
    A2["HF-cache snapshot<br/>(shards = symlinks into blobs/)"] --> B2["resolve link -> validate target<br/>(exists, regular, size bounds)"] --> C2["valid: MTP activates<br/>broken/invalid: rejection names reason + path,<br/>logged at warning"]
  end

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Code

flowchart LR
  subgraph Before
    D1[SpecDecArtifactFunnel.prepare] -->|"inspectInlineArtifact -> SpecDecArtifact?"| E1["mega-guard, nil on ANY failure"]
    F1[SpecDecStore.revalidateForLoad] -->|same nil| E1
    E1 --> G1["disabled(.inlineArtifactInvalid) — reason lost"]
  end
  subgraph After
    D2[SpecDecArtifactFunnel.prepare] -->|"inspectInlineArtifact -> Result"| E2["per-check rejections via rejectInline()<br/>reason + path, logged (darkbloom.SpecDecStore)"]
    F2[SpecDecStore.revalidateForLoad] -->|"failure detail -> fallback(detail:)"| E2
    E2 --> H2["resolvedRegularFileSize / isDirectoryResolvingSymlinks<br/>(inline path only; local overrides stay symlink-rejecting)"]
  end

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Tests

SpecDecArtifactFunnelTests — 17 tests green (swift test --filter SpecDecArtifactFunnelTests), plus 81 tests across the 7 adjacent SpecDec/MTP suites (SpecDec|ProviderMTPFactory|MTPPostureTelemetry):

• HF-cache symlinked snapshot passes inspection and admits inline MTP — builds a real blobs/ + snapshots/<rev>/ layout with relative symlinks, then proves the full activation chain short of weight loading: direct inspection succeeds with facts byte-identical to the regular-file baseline (revision, payload bytes, config/index digests), the funnel admits it as .candidate (the status the slot factory consumes), and revalidateForLoad — the last gate before assistant construction in EngineV2SlotFactory+MTP — resolves.
• Broken symlinked shard is rejected with the concrete reason and path — the Result failure names broken symlink and the shard path; the funnel maps it to the existing fail-open inline_artifact_invalid status.
• Regular-file inline inspection facts and revalidation are unchanged, and a checkpoint without inline tensors reports no tensors with prefix instead of a bare nil.

Warning logs observed in the test run, e.g.:

warning darkbloom.SpecDecStore: inline MTP artifact rejected: inline shard is missing, not a regular file, or a broken symlink: …/snapshots/0123abcd/model-00001-of-00001.safetensors

Bug 2 from the same inventory (MTP acceptance unobservable outside Datadog Logs) is fixed separately in #619.

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Project	Deployment	Actions	Updated (UTC)
d-inference	Ready	Preview	Aug 13, 2026 11:30pm
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