docs: fix ADA headless export name

[DouglasBashir]

Objective

Correct the ADA-3 submission-contract table so the documented headless export matches the challenge repository template and submission validator.

Boundaries

• Changes only docs/challenges/ada_detection/v3.md.
• Does not change challenge runtime, scoring, deployment configuration, or submission behavior.

Evidence

The ADA challenge repository defines detect_headless in the example submission, runtime template, and validation script. detect_headless_non_ua belongs to AB Sniffer and does not appear in the ADA implementation.

Validation

• git diff --check
• Corrected table checked against the ADA template and validator
• Confirmed the stale identifier no longer appears on the ADA-3 page
• Site build not run because mkdocs is not installed in the isolated workspace

Architecture and operational impact

None. This is a documentation-only contract correction.

Risk and rollback

Low risk. Revert the one-line documentation change if the ADA runtime contract is intentionally renamed in a future release.