Module `path:` subdirectory support (`ModuleConfig.path`, `wvr module add --path`)

[Max-Levitskiy]

Priority: P0 · Epic: #71 · Refs: handoff open decision 3, marketplace design G4, beads design C1

Why

ModuleConfig.path is parsed but never read (config.rs, module.rs); every module
must be a git repo root. That forces a standards module into its own repository
even when it belongs next to the standard it encodes.

What

• ModuleResolver::resolve returns <cache>/<commit>/<path> when path is set;
  validates the directory exists and contains weaver.module.yaml or files/ or
  templates/; error names the available candidates otherwise.
• weaver.lock ModuleLock records path.
• wvr module add <src> --path <sub> writes it; describe --json shows it.
• The examples harness (prepare_module_sources) supports path: so an example can
  keep several modules in one local repo.

Acceptance

• Example 43-module-in-subdirectory: one local git repo with modules/a and
  modules/b; two apps consume them via path:; stage: implemented.
• Lock round-trip test with path.