feat(core): support nested repeats with item paths

[Railly]

Summary

Adds nested repeat support by allowing { "$item": "field" } in repeat.statePath. This replaces and completes #256 on current main.

Trevin Chow (@tmchow) authored the original implementation in #256 and is credited as a co-author on the commit. This PR keeps that design while addressing current-main compatibility, validation, renderer parity, and documentation gaps.

Changes

• add shared core helpers for resolving root and item-relative repeat paths
• preserve all existing string statePath behavior
• reject relative repeat paths outside an enclosing repeat scope
• support nested repeats across React, React Native, React Email, React PDF, Image, Ink, Solid, Svelte, and Vue
• validate nested sample state, DAG reuse, empty outer arrays, and cyclic child graphs
• preserve repeat in all renderer schemas with a parity regression test
• update generated prompts, package docs, web docs, and renderer skills

Example

{
  "type": "Table",
  "repeat": { "statePath": { "$item": "employees" }, "key": "id" },
  "children": ["employee-row"],
  "props": {}
}

Inside an outer repeat over /companies, this resolves to /companies/0/employees, /companies/1/employees, and so on.

Verification

• 162 focused tests pass across core, React, and Svelte
• affected package builds pass across core and all renderer families
• Svelte type-check passes with 0 errors and 0 warnings
• force-red verification confirmed the new regression tests fail without the fix
• independent cross-model review found no blocking correctness or compatibility issues

Closes #252
Supersedes #256

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