perf: optimize core function paths across indexing, search, cache, and MCP interface

[DevvGwardo]

Summary

This PR applies broad function-level optimizations and correctness hardening across Peppy’s hot paths.

What changed

1) Cache performance + correctness (peppy/cache.py)

• Switched cache keying from MD5 to SHA256.
• Added optional ttl_seconds support.
• Added index freshness signature validation (file count + size + max mtime fingerprint) to reduce stale cache usage.
• Standardized UTF-8 read/write paths.

2) Indexer optimization (peppy/indexer.py)

• Optimized ignore checks via pre-split exact vs glob ignore patterns.
• Normalized gitignore matching using relative POSIX paths.
• Added adaptive worker defaults for better CPU utilization on modern hosts.
• Precomputed/stored symbol_types and file_extensions in index metadata.

3) Searcher optimization (peppy/searcher.py)

• Added max_results support to search_symbols() with early termination.
• Improved file pattern matching reliability (relative path + basename + full path).
• Fixed get_file_symbols() relative path behavior by resolving against codebase root.
• Reduced grep overhead and builds context payload only when requested.
• Reuses precomputed stats from index when available.

4) Parser correctness fix (peppy/parsers.py)

• Fixed AST traversal context leakage so class context is scoped per branch and siblings are not misclassified.

5) Interface/MCP tool-path optimization (peppy/interface.py, peppy/server.py)

• find_symbols() now pushes limits down to search layer (max_results) instead of slicing after full search.
• Tightened overview entrypoint symbol queries for better token/latency behavior.
• Added max_results to MCP search_symbols schema and execution flow.

6) Packaging entrypoint correctness (pyproject.toml)

• Updated script entrypoint from peppy.server:main to peppy.server:run.

Why

• Lower end-to-end latency and compute overhead on large repos.
• Improve symbol/search correctness and cache freshness.
• Better behavior for Claude/MCP workflows under heavier usage.

Scope

• Backward-compatible API changes (new optional arguments and tighter internals).
• No intended breaking behavior for existing users.