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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🤖 Generated with Claude Code

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Note

Low Risk
Marketing-site client-side download URL selection only; no auth or backend changes, with a conservative x64 fallback when detection is unclear.

Overview
Fixes the marketing homepage Download for macOS link so Intel Macs get the -x64.dmg instead of always preferring -arm64.dmg.

detectPlatform() no longer hardcodes arch: "arm64" for every Mac. It calls new detectMacArch(), which reads the WebGL UNMASKED_RENDERER_WEBGL string (when WEBGL_debug_renderer_info is available) and passes it to macArchFromGpuRenderer.

That helper classifies renderers as x64 when Intel/AMD/Radeon/Nvidia markers appear (checked first, including ANGLE strings that mention both Apple and Intel), arm64 when the string matches Apple M\d, and x64 for ambiguous values like Safari’s generic “Apple GPU” or when probing fails—so Intel users aren’t offered an arm64 build that won’t run.

Unit tests cover Apple Silicon, Intel-priority, and ambiguous/empty renderer cases.

^{Reviewed by Cursor Bugbot for commit ddab7a2. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Detect Mac chip architecture on homepage download button via WebGL GPU renderer

• Previously, macOS platform detection always returned arm64; it now probes the GPU renderer via WebGL and maps it to arm64 (Apple Silicon) or x64 (Intel/AMD/etc.).
• Adds macArchFromGpuRenderer which matches Apple M<digit> strings for arm64, Intel/AMD/Radeon/Nvidia/GeForce for x64, and defaults to x64 for ambiguous values (e.g. Safari's generic 'Apple GPU').
• detectMacArch creates a canvas, queries the WEBGL_debug_renderer_info extension, and falls back to x64 if probing fails.
• Behavioral Change: macOS users on Intel Macs will now be offered the x64 download instead of arm64.

^{Macroscope summarized ddab7a2.}

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[macroscopeapp]

Approvability

Verdict: Approved 01253b7

Self-contained marketing page fix that adds Mac architecture detection for the download button. The change is well-tested, has a safe fallback (x64 for ambiguous cases), and doesn't affect any sensitive code paths.

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Dismissing prior approval to re-evaluate 01253b7