DRAFT: opt-in artist intent propagation across workflow stages (#252)

[JayceeB1]

Source and scope

Third reviewable slice of lightningpixel/modly#252, originally designed by @OmarB97. This follows our explicit Artist Context core #6. Nothing touches fork main or upstream.

New behavior

Only workflows containing Remember Artist Context (intent-travel/start) opt in. The node stamps a staged input image with a modly.intent.v1 record. The current workflow runner now carries that record after each successful image/mesh-processing stage, including model generation, optimizers and exporters, before publishing the next output.

• Persist the output marker in per-run node outputs, not persistent global state.
• Preserve original request identity and stage history by calling the existing built-in intent-travel/carry.
• Do not copy for same-path pass-through operations.
• Reject ambiguous branches when a tagged asset is combined with a different tagged source or with an untagged file-backed source.
• Support the app's drag/drop base64 image input for the start node.
• Workflows without intent-travel/start remain unchanged.

Verification

• 7 pure Node tests added for opt-in, matching roots, mixed sources, text-only edges and absent markers.
• Linux + Windows CI jobs wired for the new branch.
• Await combined test/typecheck/build on the branch.
• Not yet executed: real GPU generation workflow on Windows verifying the resulting .tags.json across all stages and handling restart/error scenarios.
• Staged image retention and cleanup policy.

Detailed contract: docs/experimental-artist-intent-propagation.md.

Draft only. This PR makes intent propagation opt-in, not silently mandatory for current workflows. Please don't merge or distribute before runtime smoke tests.