pipeline: reject resref collisions when chunking; document erf post-write hashing (#8)

## What

Two correctness fixes surfaced in review of the direct-depot HAK artifact work.

### Reject resref+type collisions in `chunksFromManifest`

A manifest can name two distinct source paths that collapse to the same
`resref+type` (e.g. `creature/foo.tga` and `placeable/foo.tga` — resref is the
lowercase basename minus extension). The ERF writer keys resources on
`Name:Type`, so the second silently shadowed the first: an asset would vanish
from the packed HAK with no error.

`chunksFromManifest` now runs `ensureUniqueChunkResources` per chunk and fails
the build on a duplicate. This guards **both** build paths — the legacy
`--source-manifest` flow and the direct content-addressed flow
(`chunksFromSourceManifest` → `chunksFromManifest`).

### Document erf post-write hash coupling

`writeResourceData` streams the source into the output while hashing, so
size/SHA mismatches are only detected *after* the bytes are written. A non-nil
return therefore means the writer holds partial, unverified output and the
caller must discard it. Added a comment making that contract explicit;
`writeHAKArchive` already honours it (writes to a temp file, removes on any
Write error, never renames a bad archive into place).

## Tests

- `TestChunksFromManifestRejectsResrefCollision`: collision → error, distinct
  resrefs → clean. Asserts only error presence/absence — silent asset loss is
  the contract, not any specific wording.
- `go vet ./internal/erf/ ./internal/pipeline/` clean.
- `go test ./internal/erf/ ./internal/pipeline/` green.

## Follow-up

A new crucible release must be cut after this merges so the guard ships in the
binary `sow-assets-manifest` pins.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Reviewed-on: #8
Co-authored-by: vickydotbat <vickydotbat@tutamail.com>
Co-committed-by: vickydotbat <vickydotbat@tutamail.com>

internal/erf/erf.go

@@ -425,6 +425,11 @@ func writeResourceData(w io.Writer, resource Resource) error {
 	}
 	defer file.Close()
 
+	// The hash is computed while streaming into w, so size/SHA mismatches are
+	// only detected AFTER the (bad) bytes have already been written. A non-nil
+	// return therefore means w holds partially-written, unverified output; the
+	// caller must discard it. writeHAKArchive does this by writing to a temp file
+	// and removing it on any Write error rather than renaming it into place.
 	hash := sha256.New()
 	written, err := io.Copy(io.MultiWriter(w, hash), file)
 	if err != nil {