P3.2: manifest model + std.json parse + validate

Define the v0 publish manifest, parse it via std.json into a typed
struct, and validate it.

- src/manifest/manifest.sx: typed Manifest { app, version, channel,
  artifacts: List(ManifestArtifact) } + ManifestArtifact { platform,
  path, filename, content_type, metadata }. parse_manifest walks the
  std.json Value tree into the struct, surfacing every malformed/
  missing/wrong-type field as a distinct typed ManifestErr (BadJson,
  WrongType, MissingField, UnknownPlatform, MissingArtifact, Io) — never
  a silent default. Reuses P2.1 parse_platform for the platform id.
  validate_manifest checks each artifact path exists on disk (fs.exists),
  resolved relative to the manifest's directory. Strings are copied into
  the caller's allocator (long-lived-container rule).
- examples/dist.json: representative valid manifest (android_apk + ios).
- examples/fixtures/: tiny stand-in artifact byte files referenced by it.
- tests/manifest_parse.sx: parses dist.json and asserts fields; asserts
  the three failure classes surface distinct typed errors.

examples/fixtures/acme-1.2.3-android.apk

@@ -0,0 +1 @@
+APK0

examples/fixtures/acme-1.2.3-ios.ipa

@@ -0,0 +1 @@
+IPA0