fix: propagate search failures and normalize Node.js package architecture

[aooohan]

Node.js search returned a successful empty list on network errors and non-200 HTTP responses, allowing vfox to cache the failure. Raise an actionable error instead, so the core skips cache writes and subsequent searches can recover.

Normalize Go's 386 architecture to Node's x86 package suffix, retaining the existing amd64 → x64 mapping and native ARM64 selection. Report missing runtime architecture or an archive absent from SHASUMS256.txt before attempting a nonexistent download. The Windows issue does not include its actual runtime architecture, so this fixes the confirmed missing mapping without forcing all Windows hosts to x64.

Validation:

• Reproduced failures before fixing; nine offline hook cases pass, now included in CI.
• Actual vfox SDK cache integration: HTTP 503 creates no cache, a fresh SDK retries successfully, and another fresh SDK reuses the successful cache.
• Windows x64/x86/ARM64 package names resolve against official Node 20.16.0 checksums. No Windows installation was performed.
• Plugin package validation and actionlint pass.

Fixes #16
Fixes #23