fix(pack): resolve embedded fs directories from the module root

[skhaz]

Why

Packing or publishing a module that ships embedded filesystems — e.g. a UI bundle and a WASM binary — could report success while embedding zero files. The embed stage resolved each fs.directory path against the process working directory instead of the module source root the runtime uses, then skipped a missing directory with only a warning while still rewriting the entry to fs.embed. The result was a .wapp whose fs.embed entries had no backing resource; installed modules served no UI and failed to load the WASM binary.

What

• New api/service/fs/directory.ResolveDirectory centralizes directory resolution (absolute / base: project / module source root); the runtime directory manager and the embed build stage now share it, so they cannot drift.
• The embed stage resolves module-relative directories against the module root (the lock loader already registers per-module source roots; publish passes the module root explicitly) and now returns an error when an embeddable directory cannot be found, instead of silently producing a broken pack.

Proof (CLI, ui+wasm module)

Before — silent success, empty pack (working dir does not contain the asset dirs):

$ wippy pack --embed ui_fs,wasm_fs out.wapp
✓ Pack created successfully    Size: 0.87 KB    exit 0
# inspecting the pack:
RESOURCES: 0
  - id=app:ui_fs    kind=fs.embed      # claims an embedded fs
  - id=app:wasm_fs  kind=fs.embed      # claims an embedded fs
# 0.87 KB — the 14 KB WASM is absent

After — fails loudly, no broken pack:

$ wippy pack --embed ui_fs,wasm_fs out.wapp
✗ Error: stage 'embed_fs' failed: embed app:ui_fs: directory "./static" not found
exit 1

After — correct layout embeds both filesystems:

$ wippy pack --embed ui_fs,wasm_fs out.wapp
✓ Pack created successfully    Size: 7.60 KB
  Embedded resources:
    • app:ui_fs   (1 files, 0.03 KB)
    • app:wasm_fs (1 files, 14.07 KB)   # WASM actually embedded
# inspecting the pack:
RESOURCES: 2
  - id=app:ui_fs    type=tree files=1 size=28
  - id=app:wasm_fs  type=tree files=1 size=14410

Testing

• New unit + integration tests (incl. an end-to-end stage→wapp.Writer→read-back that embeds and reads both the UI and WASM filesystems). Reverting each fix facet makes the matching test fail.
• golangci-lint clean; go vet clean; full module build clean; full short race test suite green.
• Mutation testing (gremlins): 100% on both changed packages.