Signal extensions plugin architecture for QualitySignal

[grparry]

Proposal to add a generic extension mechanism to QualitySignal so consumers can attach domain-specific structured data without upstream changes.

See ADR-045 in PR #204 for the full design.

(Previously tracked in PR #197, which was superseded by #204 after branch rename.)

[grparry]

PR #197 now includes a proposed implementation alongside the ADR:

• Rust: extensions field on QualitySignal, SignalExtensionHandler trait, handler registration and dispatch on IntelligenceLoader, 7 new tests (all passing)
• TypeScript: matching interfaces and IntelligenceLoader updates, FileSignalProvider passthrough

Everything is backward compatible — existing signals, providers, and consumers work unchanged.

[grparry]

Correction: PR #197 was closed and replaced by PR #204 (branch renamed from feat/adr-044-signal-extensions to feat/adr-045-signal-extensions). All implementation work is now on #204.