feat: select fastest install mirrors

[njzjz-bot]

Problem

• dp1s currently picks conda/pixi mirrors only from coarse location detection.
• Users in China or behind unusual networks can still hit a slow or unavailable endpoint.

Change

• Add lightweight parallel benchmarking for conda channel mirrors and pixi repository mirrors.
• Cache the selected mirror for 24 hours, with fallbacks that preserve the existing CN/non-CN behavior.
• Add environment overrides for disabling auto-selection, customizing mirror candidates, and forcing conda/pixi endpoints.
• Cover the new selection logic with a mocked GitHub Actions test and document the new options.

Notes

• The existing channel-argument tests explicitly set DP1S_CHANNEL_AUTO=0 to keep their original fallback expectations deterministic.

Authored by OpenClaw (model: custom-chat-jinzhezeng-group/gpt-5.5)