feat(core): improve describe element grounding

[fishear]

Summary

• add structured describe primitives for dropdowns, options, controls, status/text targets, and diagnostic retry metadata
• tighten describe/locate prompts so field bodies, dropdown values/options, icons, and repeated table/list targets are separated more reliably
• add diagnostic retry feedback centered on marker 1 context while treating marker 2 as the failed locator result
• update describe deep-locate marker/crop handling and Qwen3 bbox normalization used by grounding verification

Evaluation

• full none run: verify-20260625-214738
• full diagnostic run: verify-20260625-215733
• targeted FCST diagnostic follow-up: verify-20260625-221223 passed; verify-20260625-221801 remained flaky on the shadow screenshot, so this case is not counted as a stable fix

Validation

• pnpm --dir packages/core exec vitest --run -u tests/unit-test/prompt/describe.test.ts tests/unit-test/agent-describe-element.test.ts
• pnpm run lint

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