1.5.0: arm64 + dynamic CloudFront source URL (LOC-6563)

[yashdsaraf]

Summary

• Linux arm64 support — RbConfig::CONFIG['host_cpu'] =~ /arm64|aarch64/ selects BrowserStackLocal-linux-arm64. Branch order mirrors the Node SDK (arm64 wins over alpine on musl).
• Dynamic source URL — new BrowserStack::FetchDownloadSourceUrl POSTs to local.browserstack.com/binary/api/v1/endpoint to discover the current CDN base. CloudFront is the primary; CloudFlare is requested as fallback on the 5th-remaining retry via X-Local-Fallback-Cloudflare: true header + error_message body field. Joins the Node SDK pattern that has been in prod for years.
• Proxy passthrough on binary download — proxyHost/proxyPort from Local#start are now also threaded into the endpoint POST and binary GET. Users behind a forward proxy can now download (was: proxy was only applied to the running binary, not the download).
• User-Agent — browserstack-local-ruby/<version> header on both the endpoint POST and the binary GET. Helps server-side telemetry attribute downloads by binding+version.
• TLS certificate verification now enforced (OpenSSL::SSL::VERIFY_PEER). Previous releases used VERIFY_NONE. The new POST carries the user's auth_token — shipping that under VERIFY_NONE would have been a security regression. CHANGELOG calls this out; users relying on disabled verification can pin to 1.4.3.

Background

Tracks LOC-6563. C# parity PR ships in coordination — see browserstack-local-csharp #LOC-6563.

Files

• New: `lib/browserstack/fetch_download_source_url.rb`, `lib/browserstack/version.rb`
• Rewritten: `lib/browserstack/localbinary.rb` (arm64, dynamic URL, retry-with-fallback, proxy, VERIFY_PEER, UA)
• Modified: `lib/browserstack/local.rb` (threads conf into LocalBinary.new), `browserstack-local.gemspec` (1.4.3 → 1.5.0 + files list), `CHANGELOG.md` (1.5.0 entry with VERIFY_PEER callout), `test/browserstack-local-test.rb` (8 new unit tests)

Test plan

• `ruby -Ilib -Itest test/browserstack-local-test.rb -n '/BrowserStackLocalBinaryTest/'` — 8 new unit tests pass (arm64 gnu/musl, alpine, darwin arm64, default + custom UA, proxy passthrough)
• Manual on darwin: `ruby -Ilib -e 'require "browserstack/local"; b = BrowserStack::Local.new(ENV["BROWSERSTACK_ACCESS_KEY"]); b.start; puts "pid=#{b.pid}"; sleep 1; b.stop'` downloads + verifies
• Manual on Linux arm64 — picks `BrowserStackLocal-linux-arm64`
• Manual behind a forward proxy — `Local.new.start({"proxyHost" => ..., "proxyPort" => ...})` downloads via proxy
• Travis / existing integration tests pass with a real `BROWSERSTACK_ACCESS_KEY`

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