fix(marketing): detect Mac chip on homepage download button

[mahdibm-dev]

Problem

On an Intel Mac, the hero Download for macOS button on t3.codes serves the arm64 DMG, which won't launch — macOS reports "can't run on this Mac" (Intel Macs can't run arm64 binaries, and Rosetta only translates the other direction).

In apps/marketing/src/pages/index.astro, detectPlatform() hardcoded arch: "arm64" for all Macs, so pickAsset() always preferred -arm64.dmg. navigator.userAgent reports "Intel Mac OS X" on both Intel and Apple Silicon Macs, so the chip can't be told apart from the UA string alone.

Closes #4196

Fix

Add detectMacArch(), which probes the GPU via the WebGL WEBGL_debug_renderer_info renderer string:

• renderer contains "Apple" → arm64 (Apple Silicon)
• renderer contains intel/amd/radeon/nvidia/geforce → x64 (Intel)
• inconclusive / WebGL unavailable → default to arm64 (matches prior behavior; most current Macs are Apple Silicon)

detectPlatform() now calls it instead of hardcoding the arch. The /download page already lists both builds explicitly, so it needed no change.

UI note (per CONTRIBUTING.md)

This is a behavior-only change — the button's text and styling are unchanged, so before/after screenshots would be identical. Only the download URL it resolves to changes:

Machine	Before	After
Intel Mac	T3-Code-*-arm64.dmg (won't launch)	T3-Code-*-x64.dmg
Apple Silicon	T3-Code-*-arm64.dmg	T3-Code-*-arm64.dmg (unchanged)

Testing

• pnpm --filter @t3tools/marketing typecheck (astro check) → 0 errors, 0 warnings, 0 hints.
• On an Intel Mac (MacBookPro16,1), the WebGL renderer resolves to an Intel GPU, so the button now points at the -x64.dmg asset; Apple Silicon still resolves to -arm64.dmg.

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[macroscopeapp]

Approvability

Verdict: Approved

Straightforward bug fix that adds Mac chip detection for the homepage download button. The change is self-contained to the marketing page, has proper error handling, and carries minimal risk.

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