[Core] Two-phase pipeline + parallel rendering (+ incremental)

[jeffpatton1971]

Summary
Build symbol index once; render per type in parallel with deterministic outputs. Add hash/timestamp-based skip to avoid rewriting identical files.

Tasks

• SymbolIndex.Build(...) phase in Core.
• RendererRunner with Parallelism, DiffOnly, DryRun, ReportPath.
• Hash/timestamp checks for incremental skip.

Acceptance

• Deterministic outputs; measurable speedup on sample.
• Report file (paths written/skipped, timings) when requested.