fix: scope repeating group validation errors to the invalid instance (#1985)

[clinnygee]

Fixes #1985

Problem

Validation errors on repeating group items showed on all instances when only one was invalid. Errors were stored/looked up in a flat Record<linkId, OperationOutcome>, and every field looked itself up as invalidItems[qItem.linkId]. Because all instances of a repeat-group child share one linkId, they collided on a single key — one invalid instance lit up all of them. The save gate was unaffected; this was purely a display issue.

Fix

Errors are now keyed by linkId plus the path of enclosing repeat-group instance indices (e.g. child-field///1). Non-repeating items have an empty path, so their key stays a bare linkId — no behaviour change.

Validation (a pure function over the QR) and the display hook build the identical key from one shared helper, so they always agree without mutating or persisting anything into the QuestionnaireResponse. The instance-index path reaches descendants through a small React context provided per repeat instance / gtable row.

• New utils/validateErrorKey.ts — getValidationErrorKey() / getBaseLinkIdFromErrorKey()
• New contexts/RepeatGroupInstanceContext.ts — carries the instance-index path
• utils/validate.ts — threads repeatInstancePath through the recursion and keys all three error write sites via the helper
• RepeatGroupItem.tsx and GroupTableRow.tsx — provide their index path via the context
• useValidationFeedback.ts — reads the context and looks up the instance-scoped key (no signature change, so the ~25 leaf components are untouched)
• SaveAsFinalAction.tsx — maps error keys back to base linkIds so "jump to first error tab" still resolves

Why not the literal qrItem.id ?? linkId from the issue?

The primary reported case is an empty required field, which has no qrItem at the leaf (it arrives as undefined) — so there'd be nothing to carry an id. A position-path key handles that case, needs no QR mutation/stripping, and composes for nested repeat groups.

Out of scope (follow-ups)

• targetConstraint per-instance validation (explicitly deferred in the issue)
• Repeating single (non-group) items — still keyed by linkId

Nested repeat groups and gtable rows are covered by this change.

Testing

• New test/validateErrorKey.test.ts (pure helper) and test/validateRepeatInstance.test.ts (end-to-end validateForm: only the empty instance is flagged, not the answered one nor the bare linkId)
• Full renderer suite: 2179 passed
• Renderer npm run build: clean · npm run lint: 0 errors · app tsc --noEmit: clean

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