fix: adversarial review round 6 — release v0.1.8

[ejwhite7]

Fixes all 13 findings from review round 6:

High:

• Honor server Retry-After on 429 (capped at 60s) instead of always using local exponential backoff (base_provider.rb)
• Parallel executor timeout/rejection Results carry the real tool name instead of "unknown"; rejected futures no longer report a misleading full-timeout duration (executor.rb)
• Tool blocks with keyword parameters now work — arguments are splatted to keywords when the block declares them (definition.rb)

Medium:

• Wire the Loop's emitter into Compaction so the documented :compaction event actually fires (loop.rb)
• Buffer SSE chunks as BINARY and re-encode complete lines to UTF-8 in all three providers; streaming deltas are now always valid UTF-8
• :fallback_start payload includes partial_output/partial_chars so consumers can truncate partial primary text (fallback.rb)
• Validate tool names against provider constraints at definition time
• Declare json as a runtime dependency and require it in every file that uses it (composability; pinned by a source-scan spec)

Low/docs:

• Configuration rejects negative numeric settings; eager-init the global configuration to remove the first-access race
• Document continue() per-invocation Result accounting, schema-as-hints (not validation), and unbounded message growth without compaction
• Correct CLAUDE.md module map (agent/agent.rb never existed), version reference, and extension handler signature
• CHANGELOG note: 0.1.4 was never actually released (version.rb went 0.1.3 → 0.1.5); created the missing v0.1.7 tag locally

22 new regression specs in spec/ruby_pi/fixes/review_round6_spec.rb. Suite: 492 examples, 0 failures.