Correct the README project-structure tree

[tschm]

Subcategory: user-facing documentation
Score: 6 → 9

Problem

The README's "Project structure" section describes a layout this repo does not
have. README.md:250-259:

pycharting/
├── src/
│   ├── core/         # Chart server lifecycle and internals
│   ├── data/         # Data ingestion, validation, and slicing
│   ├── api/          # FastAPI routes and Python API surface
│   └── web/          # Static frontend (HTML + JS for charts)
├── tests/            # Test suite
├── data/             # Sample CSVs and fixtures
└── pyproject.toml    # Project configuration

Two things are wrong:

1. The packages live at src/pycharting/core/, src/pycharting/data/,
   src/pycharting/api/, src/pycharting/web/ — not directly under src/.
   The section's own opening line ("The library follows a modern src/ layout")
   is correct; the tree contradicts it.
2. There is no top-level data/ directory. ls data → No such file or directory.

The same fence is the one flagged by /rhiza:quality's gate 9 as carrying no
language tag, so no tooling can check it either:

"1 fence(s) carry no language, so nothing can check them: line 250"

Files to change

• README.md:250-259

done when

• The tree reflects the real layout (src/pycharting/...) and drops the
  non-existent data/ entry.
• The fence is tagged ```text so check_doc_examples.py reports no untagged
  fences.

Evidence

check_doc_examples.py --source-root src --json → README block at line 250,
"status": "untagged". Directory listing confirms src/pycharting/{api,core,data,web}
and no top-level data/.

Found by /rhiza:quality (full mode, rhiza v0.18.8).