Add Binxtils::Secure for constant-time secret comparison

[sethherr]

Adds a functionable Binxtils::Secure module for comparing a value against an expected secret (API tokens, webhook signatures) without leaking length or content through timing.

• compare?(value, expected) — returns false unless expected is present, then defers to ActiveSupport::SecurityUtils.secure_compare. Both sides are to_s'd, so a non-string secret (e.g. an integer from config) compares rather than raising NoMethodError on bytesize.
• lib/binxtils.rb requires active_support/security_utils explicitly — the existing active_support/core_ext require doesn't pull it in.
• Specs cover match, length mismatch, single-character mismatch, non-string value/expected, and blank expected.