docs(benchmark): use PNG frontier chart so GitHub renders it in README

GitHub does not reliably render inline SVG via <img src>; all other README
images are PNG. Switch the v2 frontier comparison chart to a PNG (dark-theme
bar chart, 8 models by suite base score) referenced from the root README,
benchmark README, and leaderboard.

Author: NeoXider

Assets/CoreAIBenchmark/README.md

@@ -223,7 +223,7 @@ Frontier hosted models, run through the [`cli-agents`](https://github.com/) `ope
 OpenAI-compatible shim over the Claude Code / Codex CLIs. This is a **separate leaderboard** from the local
 LM Studio models above (different suite version *and* different backend class). Ranked by suite base score.
 
-![CoreAI Game-Creation Benchmark v2 — frontier-model comparison](../../Docs/Images/benchmark_v2_frontier.svg)
+![CoreAI Game-Creation Benchmark v2 — frontier-model comparison](../../Docs/Images/benchmark_v2_frontier.png)
 
 | # | Model | Score | Pass-rate | P/PA/F | G1 | G2 | G3 | G4 | G5 | G6 | G7 | G8 | Tokens |
 |---:|---|---:|---:|---|---:|---:|---:|---:|---:|---:|---:|---:|---:|

Docs/BENCHMARK_LEADERBOARD.md

@@ -25,7 +25,7 @@ Scores are only comparable **within the same suite version**. Scenario sets, che
 
 ### Frontier models (2026-07-11 maintainer sweep, G1–G8)
 
-<img src="Images/benchmark_v2_frontier.svg" alt="CoreAI Game-Creation Benchmark v2 — frontier-model comparison (suite base score, TerminalBench-style bar chart)" width="900">
+<img src="Images/benchmark_v2_frontier.png" alt="CoreAI Game-Creation Benchmark v2 — frontier-model comparison (suite base score, TerminalBench-style bar chart)" width="900">
 
 Run through the `cli-agents` `openai-server` bridge (an OpenAI-compatible shim over the Claude Code / Codex
 CLIs). Ranked by suite base score. Per-group scores are G1…G8; see the benchmark README for the group legend.

Docs/Images/benchmark_v2_frontier.png

@@ -89,7 +89,7 @@ CoreAI ships a game-creation benchmark: it scores how well an LLM builds and cha
 
 **Top-tier models — v2 frontier sweep (2026-07-11):**
 
-<img src="Docs/Images/benchmark_v2_frontier.svg" alt="CoreAI Game-Creation Benchmark v2 — top-tier frontier-model comparison ranked by suite score" width="900">
+<img src="Docs/Images/benchmark_v2_frontier.png" alt="CoreAI Game-Creation Benchmark v2 — top-tier frontier-model comparison ranked by suite score" width="900">
 
 | # | Model | Suite | Pass-rate |
 |---:|---|---:|---:|