fix: reflect interface fields in VML repeaters

[medvednikov]

Fixes #26.

Changes

Add interface-aware VML value and schema conversion. Runtime values expose the public fields declared by the interface; schema construction uses each declared field's type rather than selecting fields from a zero interface, so an empty []Item can still validate safely.

Regression coverage includes mixed value-backed and pointer-backed implementations, item.id keys and item.label rendering, action arguments, stable event identities after reordering, duplicate-key rejection, empty collections, and rejection of fields outside the interface contract.

Verification

Passed with V 0.5.2:

v -d ui2_headless test ui/vml_interface_repeater_test.v
v -d ui2_headless test ui/vml_model_test.v

The new regression fails with cannot read id from item.id against the original code and passes with this change. Both commands also passed on Linux before branch publication: verification log. That shared run subsequently failed on an existing scrolling-test compilation error while checking a different issue.

The interface regression tests also passed locally under AddressSanitizer (-cc gcc -gc none, leak detection disabled). This is an independent PR against master and includes no temporary verification workflow.

[chatgpt-codex-connector]

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-19T14:05:31.409207Z	a90f40c	PR opened

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[chatgpt-codex-connector]

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Reviewed commit: a90f40ce8a

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[chatgpt-codex-connector]

Guard zero interface values before reflecting fields

When a public model field uses an interface with declared fields but is left at its zero value, v_value_from eagerly converts that field even if the VML never references it, and this selector dereferences an interface with no backing object. That makes otherwise unrelated template rendering crash; previously, unsupported interface fields became an invalid VValue without being dereferenced. Detect a zero/nil-backed interface and return an invalid value or an actionable error before selecting its fields.

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