Add per-instance Net::HTTP Semian policy

[serioushaircut]

Summary

Add an explicit per-instance policy to the Net::HTTP adapter so callers can inherit the process-wide configuration, disable Semian, or replace the whole resource with caller-owned options. This lets HTTP client frameworks configure an already-created Net::HTTP object before its first Semian lookup.

Approach

• Preserve omitted semian: and semian: true as inheritance from Semian::NetHTTP.semian_configuration
• Treat semian: false as a complete opt-out and a hash as a complete replacement, without merging platform circuit-breaker or bulkhead settings
• Expose semian_policy= for pre-use framework integration, defensively copy and freeze caller options, allow idempotent repeats, and reject policy changes after assignment or use
• Synchronize policy assignment with first use so concurrent callers cannot change protection after lookup begins
• Give disabled instances a stable unprotected resource identity and cache the explicit opt-out path
• Validate caller-owned names and reject dynamic: true for immutable instance policies
• Document whole-resource replacement, bulkhead implications, and the constructor limitation for subclasses that perform Semian work during initialization

Validation

• bundle exec rubocop (134 files, no offenses)
• Focused policy tests (14 runs, 61 assertions, 1 Linux-only bulkhead test skipped locally)
• Net::HTTP adapter tests excluding the existing macOS-incompatible SysV bulkhead assertion (41 runs, 150 assertions, 1 Linux-only positive bulkhead test skipped locally)

Co-authored-by: GPT-5.6-sol noreply@openai.com
Orchestrated-by: ae noreply@shopify.com