perf: update and optimize core functions end-to-end (cache/index/search/interface/server)

[DevvGwardo]

Summary

This PR performs a broad optimization + correctness pass across Peppy core functions and hardens related tests.

Phase results

• Parallel analysis completed (structure + code scouts)
• Changes synthesized and applied to high-impact runtime paths
• Branch created and files committed
• PR opened immediately

Core code updates

peppy/cache.py

• Upgraded cache key hash to sha256.
• Added optional ttl_seconds support.
• Added cache freshness signature checks against indexed files to reduce stale index reuse.
• Standardized UTF-8 read/write handling.

peppy/indexer.py

• Optimized ignore logic by splitting exact patterns vs glob patterns.
• Normalized gitignore checks against relative POSIX paths.
• Added adaptive worker sizing for indexing throughput.
• Precomputes/stores symbol_types and file_extensions in index metadata.

peppy/searcher.py

• Added max_results to search_symbols() with early termination.
• Improved file pattern matching (relative path, basename, and full path checks).
• Fixed get_file_symbols() so relative file paths resolve from codebase root.
• Reduced grep overhead and only builds context payload when requested.
• Reuses precomputed stats when available.

peppy/parsers.py

• Fixed AST traversal context leakage by scoping class context per branch (prevents sibling misclassification).

peppy/interface.py

• Pushes symbol limits down to search layer (max_results) instead of slicing after full scan.
• Tightened overview entrypoint query patterns and caps for lower overhead.
• Reuses precomputed index stats when present.

peppy/server.py

• Added max_results to MCP search_symbols schema.
• Wired max_results into runtime search execution.

pyproject.toml

• Corrected CLI entrypoint from peppy.server:main to peppy.server:run.

Test hardening

tests/unit/test_parsers.py

• Fixed no-op assertion (classes[0].name == ... -> real assert).
• Removed effectively-always-true assertion and replaced with meaningful symbol assertion.

tests/unit/test_searcher.py

• Updated symbol-limit test to validate real max_results behavior.
• Added regression test for relative file path resolution against codebase root.

Why

• Improve runtime performance and scalability on larger codebases.
• Fix correctness gaps in path resolution, parser classification, and cache freshness.
• Improve confidence via stronger regression tests.

Compatibility

• Changes are backward-compatible; new parameters are optional.
• Existing workflows should continue to function with improved behavior.