fix(index): accept a single file as a source path

[Sylvere-bamenou]

Problem

Following the README's own PDF quickstart end to end fails:

source:
  type: local
  path: ./paper.pdf

LocalSource discovers files with rglob("*") and PDFSource with glob("**/*.pdf"). Both return nothing when the path points at a file rather than a directory, so the source yields zero documents. Nothing errors at that point — the run continues through render, chunk and embed as no-ops and dies four stages later:

Stage 1/4: Rendering 0 documents to tiles...
Stage 4/4: Building FAISS index (0 vectors, nlist=1)...
No shard files found!
subprocess.CalledProcessError: ... returned non-zero exit status 1

The failure message points at the index builder, several stages away from the actual cause.

Reproducer against the repo's own asset:

>>> len(PDFSource("assets/pixelrag-paper.pdf")._files)
0
>>> len(PDFSource("assets")._files)
1

Fix

Branch on is_file() in both adapters, so a path pointing straight at a document is a single-item source. Directory traversal is untouched, and the single-file path keeps the same extension filtering the directory path applies.

Also

The PDF quickstart installs pixelrag[index], which cannot render a PDF — pdf2image lives in the pdf extra and needs poppler. Added to the install line and a short note.

Tests

Four tests in tests/test_source_single_file.py, including one pinning the existing directory-traversal behaviour (recursive, both adapters) so this stays a widening and not a change.

Verification

On macOS (Apple Silicon, MPS), pixelrag index build against assets/pixelrag-paper.pdf:

Source path	Before	After
assets/ (directory)	35 vectors	35 vectors
assets/pixelrag-paper.pdf (file)	0 documents, crash	35 vectors

Both indexes serve and return the expected tile — querying "Overview of PixelRAG and the diagram" returns page 2 (the Figure 1 overview), matching the README's stated expectation.

pytest tests/ → 86 passed, 6 skipped. ruff check and ruff format --check clean.

Note for reviewers

LocalSource also feeds the web/kiwix paths downstream. The existing suite only covered directory inputs, so I could not verify against a usage that relies on a file path yielding zero documents — worth a look if any exists.

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