Fix standalone URL-safe Base64 decoding

[OskarEichler]

Summary

• define urlsafe_decode64 on the fallback HTTP2::Base64 module
• keep the Ruby 3.3+ no-stdlib fallback self-contained
• restore h2c server upgrade when no other dependency preloads ::Base64

Reproduction

On Ruby 4 with gems disabled, requiring http/2/base64 defines HTTP2::Base64.urlsafe_encode64 but not its decoder. HTTP2::Server#upgrade therefore raises NoMethodError unless something else happened to load the standalone base64 gem.

Verification

• 448 examples, 0 failures on Ruby 4.0.6
• clean --disable=gems URL-safe binary round-trip
• bodyless h2c upgrade model
• all 18 runtime files pass syntax checks
• RuboCop passes the runtime source

No repository tests were changed. I used an AI coding assistant during investigation and verification.