chore(patterns): Enforce I2U coverage (per existing template)

[Meyanis95]

Context

The pattern template requires "How guarantees differ in I2I vs I2U contexts (if applicable)" in the Guarantees section (_template.md, line 33). CROPS frontmatter and CROPS context sections have been added since the last audit, but the Guarantees and Protocol sections still need a pass to ensure I2U coverage is complete across all context: both patterns.

Per-pattern checklist

For every context: both pattern:

• Guarantees section states what the end user gets (not just the institution)
• Trade-offs CROPS context has separate I2I and I2U analysis (not a one-liner)
• Protocol section notes where I2U flow diverges, if it does (key management, exit paths)
• If a pattern can't serve I2U at all, update frontmatter to context: i2i

What the template requires but most patterns don't deliver

• Guarantees: Template says to document I2I vs I2U differences. Most patterns write Guarantees from the I2I perspective only.
• Trade-offs / CROPS context: Most patterns have a single-line I2U caveat ("In I2U, end-users face greater exposure..."). Should be a substantive paragraph per context.

Scope

~48 patterns with context: both. Can be done during the QA audit pass or as a focused pre-pass. Identity patterns (~20) are a priority subset.

Done when

Every context: both pattern passes the 4-point checklist above.

[Meyanis95]

Closed by v2 pattern schema (#152, #150). I2U coverage is now enforced structurally via frontmatter: context_differentiation (required when context: both) renders the I2I-vs-I2U callout, and crops_context carries per-dimension narrative. Strict validator (#150) gates this on every pattern.