[pengpengyi92]

Summary

Fixes #391.

Add a Graph Memoization section and TOC entry to the graph concepts page:

• distinguish wiring-time sharing from caching values on each tick;
• explain node/graph memoize=False, scoped csp.memoize(False), and the
  important non-recursive graph opt-out behavior;
• describe state sharing, normal cache/run lifetime, force_memoize and factory
  object considerations;
• include a self-contained example that checks shared versus independent wiring
  and actually runs both counters.

Scope: one documentation file, 57 additions. No runtime, API, dependency or
native-code change. This is independent of the configured-realtime docs in #788.

Validation

• Exact new Markdown example extracted and executed in a fresh Python process.
• Local memoization contract probes: 13 passed. These include wiring identity,
  runtime node start counts, per-node/scoped opt-out, non-recursive graph opt-out,
  force_memoize, fresh state across runs/reused graph structure, helper-object
  sharing and unhashable-argument fallback. The extra probes are local verification,
  not new upstream test files.
• Existing tests before and after the doc change:
  python -m pytest csp/tests/test_graph_mem_cache.py csp/tests/impl/wiring csp/tests/test_engine.py::TestEngine::test_realtime_flag -q -rs
  -> 28 passed, 2 skipped (Numba unavailable).
• python -m mdformat --check docs/wiki/concepts/CSP-Graph.md -> passed.
• python -m codespell_lib docs/wiki/concepts/CSP-Graph.md -> passed.
• git diff --check -> passed.

Windows / Python 3.12.14; checked-out Python sources with official CSP 0.19.1
Windows wheel native libraries. No C++ rebuild, Linux/macOS matrix, full
service-dependent suite or performance benchmark was run. Local evidence does
not imply upstream CI success or maintainer acceptance.

Draft for maintainer scope/wording review; no engine behavior change proposed.