Implement stricter parsing to avoid skipping invalid lines

[a2Fsa2k]

Summary

Adds strict mode to the openui-lang parser so that invalid lines (lines
that don't match identifier = expression) are reported as parse-failed
errors instead of being silently skipped. Useful for debugging when LLMs
generate preamble text alongside valid openui-lang code.

Changes

• tokens.ts — Added tokenText() utility to reconstruct text from a token
• statements.ts — split() now accepts an optional skipped array to
  collect skipped line text
• parser.ts — Added strict parameter to parse(), and new
  createStrictParser() API that surfaces skipped lines as errors
• types.ts — Added "parse-failed" to ValidationErrorCode
• Tests — 5 new test cases for strict mode

Example

const parser = createStrictParser(library.toJSONSchema());
const result = parser.parse('Here is the response:\nroot = Stack([Title("hi")])');
console.log(result.meta.errors);
// [{ code: "parse-failed", message: 'Unexpected text: "Hereistheresponse:" ...' }]

Closes #582