Support multi-layer image resolution in ww run

[lthibault]

Summary

Extend ww run to accept multiple positional arguments, each a path (local filesystem or IPFS-family). The runtime merges them into a unified FHS root via per-file union before loading bin/main.wasm.

ww run <path> [<path> ...]

Later arguments override earlier arguments at the file level. Directories merge, files replace. No deletes.

Motivation

This enables overlay patterns where a base image provides defaults and overlays customize specific parts (e.g. swap boot/ peers for local dev without forking the whole image). It also lays the groundwork for --stem integration, where the contract's head CID becomes the base layer.

Design

See docs/architecture.md (on feat/ww-run-image) — "Configuration" and "Layer resolution" sections.

Key rules:

• Per-file union: each layer contributes files, later layers win on conflict
• Directories merge, files replace
• No deletes — to remove something from a lower layer, publish a new version without it
• The union must contain bin/main.wasm — no single layer needs to be complete

Implementation sketch

• Change ww run's image: String to images: Vec<String> (one or more positional args)
• Add a UnionLoader (or LayeredLoader) to src/loaders.rs that stacks multiple image roots and resolves paths top-down
• CellBuilder takes the merged root instead of a single path
• Unit tests for layer resolution (override, merge, missing bin/main.wasm error)

Depends on

• Wire pid0 into ww run <image>, remove exec + TCP RPC #32 (ww run <image>)

[lthibault]

Implementation approach for multi-layer resolution:

• Materialize the layer union on the host filesystem (temp dir). Single local layer skips the copy.
• Local layers: recursive walk + copy (walkdir). IPFS layers: kubo /api/v0/get TAR extraction.
• Later layers win on file conflict. Directories merge. No deletes.
• Mount the merged root as a read-only WASI preopen at / so guests can read boot/, svc/, etc/.
• CLI changes from image: String to images: Vec<String>.

PR will target staging.

[lthibault]

Closed by PR #40 (merged into staging).