Relabel post-manifest-parse failure as TooNew; refuse zero-length package entries at encode

An additively-tagged newer package that fails to parse now reports TooNew (with
writer semver) instead of unactionable Malformed. Format layer also refuses encoding
a zero-length entry, honoring the shell's appendPayload contract; both test-covered.

src/core/package/CLAUDE.md

@@ -79,8 +79,22 @@ landing after the format.
   version pair classifies `Readable`; for `TooNew` it stops at the semver —
   don't "fix" it to read the manifest length there, a future structural format
   may have moved it.
+- A package whose header classifies `Readable` (fv > ours, minReader still
+  within reach — the additive case) but whose manifest fails to parse is
+  reported `TooNew`, not `Malformed`: the header is valid and already carries
+  the writer's semver, so the refusal can still name what to install. This
+  widens `TooNew` to cover "read and failed" as well as "stopped at the frozen
+  region" — both refuse whole and write nothing, so the safety property is
+  unchanged, only the message. `classifyPackageVersion` and the frozen-region
+  `TooNew` path are unaffected; this is the post-manifest-parse branch only.
 - The format carries no algorithm tag for `byteHash` — it is FNV-1a
   (`capture::hashBytes`) implicitly. Changing the digest algorithm is a
   `minReaderVersion` bump, not additive: an old reader would otherwise compare
   a stored digest against bytes hashed the new way and silently misjudge
   corruption.
+- **Cross-module contract with `src/shell/package`:** a genuinely zero-length
+  entry cannot round-trip through the filesystem seam there (`appendPayload`
+  refuses an empty payload — an empty buffer signals an upstream read failure,
+  not a real entry). `serializeManifest` refuses a zero-length `PackageEntry`
+  at encode so this layer never produces one; decode does not enforce it (a
+  hostile/older package declaring one is not this track's concern).