refactor(installer): add reproducible build sources - #161

Summary

• split embedded installer payloads into shared and platform-specific sources
• generate install.sh and install.ps1 reproducibly with node build.js
• preserve the Windows PowerShell 5.1 UTF-8 BOM explicitly
• add node build.js --check to compatibility CI and release
• run the generated-source check on both Linux and Windows checkouts
• pin the complete build graph to LF via .gitattributes

The generated installers are byte-for-byte identical to current main; this PR does not change installation behavior.

Source layout

• src/shared/: extractor, post-processor, repatcher, proxy, universal patcher, features
• src/unix/ and src/windows/: genuinely platform-specific payloads
• src/templates/: installer shell with explicit placeholders

Unlike the stale sources in #108, the universal patcher and other identical payloads have one shared source of truth.

Validation

• node build.js --check
• drift detection negative test
• PowerShell BOM assertion
• node --check for every embedded JS payload
• bash -n install.sh
• actionlint for compat and release workflows

Supersedes #108.