Add LLM review command for MusicXML correction

New `sheet-music-ocr review` command that sends MusicXML output to an
LLM (Claude or OpenAI, configurable via --provider flag) for reviewing
and fixing common OCR errors like incorrect pitches, rhythms, key
signatures, and garbled lyrics. Uses httpx for direct API calls.

https://claude.ai/code/session_017GqUbuRDT58toRaxMtfRmf

python/sheet_music_ocr/review.py

@@ -0,0 +1,126 @@
+"""LLM-based MusicXML review and correction.
+
+Supports both Claude (Anthropic) and OpenAI APIs for reviewing
+MusicXML output from Audiveris and suggesting/applying fixes.
+"""
+
+from __future__ import annotations
+
+import enum
+import os
+from typing import TYPE_CHECKING
+
+import httpx
+
+if TYPE_CHECKING:
+    from pathlib import Path
+
+REVIEW_PROMPT = """\
+You are a music notation expert. Review the following MusicXML file produced by \
+optical music recognition (Audiveris). Look for and fix common OCR errors including:
+
+- Incorrect note pitches or durations
+- Wrong or missing key signatures, time signatures, or clefs
+- Incorrect rest durations or placements
+- Missing or incorrect accidentals
+- Wrong beam groupings or tuplets
+- Garbled or misspelled lyrics and text annotations
+- Missing or incorrect dynamic markings
+- Incorrect measure numbers or barline types
+- Voice/staff assignment errors
+
+Return ONLY the corrected MusicXML. Do not include any explanation, commentary, or \
+markdown formatting. Output the raw XML directly.
+
+Here is the MusicXML to review:
+
+"""
+
+_TIMEOUT = 300
+
+
+class LLMProvider(enum.StrEnum):
+    """Supported LLM providers."""
+
+    CLAUDE = "claude"
+    OPENAI = "openai"
+
+
+class ReviewError(Exception):
+    """Raised when LLM review fails."""
+
+
+def _get_api_key(provider: LLMProvider) -> str:
+    env_var = "ANTHROPIC_API_KEY" if provider == LLMProvider.CLAUDE else "OPENAI_API_KEY"
+    key = os.environ.get(env_var)
+    if not key:
+        msg = f"{env_var} environment variable is not set."
+        raise ReviewError(msg)
+    return key
+
+
+def _call_claude(content: str, api_key: str) -> str:
+    response = httpx.post(
+        "https://api.anthropic.com/v1/messages",
+        headers={
+            "x-api-key": api_key,
+            "anthropic-version": "2023-06-01",
+            "content-type": "application/json",
+        },
+        json={
+            "model": "claude-sonnet-4-20250514",
+            "max_tokens": 16384,
+            "messages": [{"role": "user", "content": REVIEW_PROMPT + content}],
+        },
+        timeout=_TIMEOUT,
+    )
+    if response.status_code != 200:  # noqa: PLR2004
+        msg = f"Claude API error ({response.status_code}): {response.text}"
+        raise ReviewError(msg)
+
+    data = response.json()
+    return data["content"][0]["text"]
+
+
+def _call_openai(content: str, api_key: str) -> str:
+    response = httpx.post(
+        "https://api.openai.com/v1/chat/completions",
+        headers={
+            "Authorization": f"Bearer {api_key}",
+            "Content-Type": "application/json",
+        },
+        json={
+            "model": "gpt-4o",
+            "messages": [{"role": "user", "content": REVIEW_PROMPT + content}],
+            "max_tokens": 16384,
+        },
+        timeout=_TIMEOUT,
+    )
+    if response.status_code != 200:  # noqa: PLR2004
+        msg = f"OpenAI API error ({response.status_code}): {response.text}"
+        raise ReviewError(msg)
+
+    data = response.json()
+    return data["choices"][0]["message"]["content"]
+
+
+def review_mxml(mxml_path: Path, provider: LLMProvider) -> str:
+    """Review a MusicXML file using an LLM and return corrected content.
+
+    Args:
+        mxml_path: Path to the .mxml file to review.
+        provider: Which LLM provider to use.
+
+    Returns:
+        The corrected MusicXML content as a string.
+
+    Raises:
+        ReviewError: If the API call fails or the key is missing.
+        FileNotFoundError: If the input file does not exist.
+    """
+    content = mxml_path.read_text(encoding="utf-8")
+    api_key = _get_api_key(provider)
+
+    if provider == LLMProvider.CLAUDE:
+        return _call_claude(content, api_key)
+    return _call_openai(content, api_key)