Add retry logic (10 retries, 30s delay) to all download commands in installation scripts

[Copilot]

• tools/install-base.sh: add download_file() function (curl/wget by availability), separate sedi alias from download logic, move downloads outside OS if/else, quote variables
• tools/install-unx.sh: replace download alias with function, pick curl/wget by availability; keep TEXDIR OS-based separately
• tools/install-bin-unix.sh: add download_file() function, replace OS-specific curl/wget if/else with single function call
• tools/install-bin-windows.bat: simplify to irm ... | iex one-liner

[CLAassistant]

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✅ yihui
❌ Copilot
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