docs(healthcare): add US FDA, UK NHS, and Singapore MOH regulatory variants

[imran-siddique]

Summary

Adds three jurisdiction-specific policy + TRACE record variants to the healthcare demo, showing how the same cMCP + TRACE architecture maps to different regulatory frameworks. The Cedar policy rules and TRACE record fields change per jurisdiction; the runtime architecture does not.

Variant	Jurisdiction	Regulatory focus
us-fda-samd/	United States	FDA AI/ML SaMD Action Plan -- cleared-scope enforcement (imaging modality gate), 21 CFR Part 820, HIPAA 164.312
uk-nhs/	United Kingdom	UK GDPR Art. 22 -- DSPT token enforced at call layer, MHRA AI as medical device, clinician-review gate
sg-moh/	Singapore	IMDA AI Governance Framework Tier 1/2 -- consequential-decision gate, PDPA consent reference, MOH guidelines

Updates the healthcare README with a variant index table.

What each variant includes

• policy/<name>.cedar -- Cedar policy bundle with jurisdiction-specific forbid rules and @regulation annotations
• trace-output/example-<name>-trust-record.json -- TRACE Trust Record with runtime.region, runtime.provider, and compliance_domains_touched set for that jurisdiction
• README.md -- what the policy enforces, key TRACE fields for that regulator, and a cross-reference table

Key policy differences

• US FDA: out-of-scope-modality forbid rule blocks inference on imaging types outside the device's cleared indication of use -- unique to this jurisdiction
• UK NHS: DSPT access token is a runtime gate on every PHI call (dspt-required forbid), not just a configuration check; UK GDPR Art. 22 uses a "clinical significance" field rather than EU's risk-category model
• Singapore: imda_tier is an explicit context field on every call; Tier 1 (consequential) requires a human_review_token unconditionally; PDPA consent reference enforced at call time, not just data collection

Relationship to existing demo

The base demo (healthcare/README.md) covers EU AI Act Art. 14 + HIPAA and remains unchanged. These variants are additive subdirectories following the same file structure.