Split docs source capture helpers

[yvette-carlisle]

Summary

• Split docs source-capture helper logic into docs/source_capture/helpers.rs.
• Kept the existing source_capture.rs facade API unchanged for callers.
• Added helper-module tests for origin fallback, identity shape, and write-policy span ordering.

Validation

• cargo make fmt-rust
• cargo make fmt-rust-check
• cargo make check-rust
• cargo make lint-vstyle
• git diff --check
• cargo nextest run -p elf-service source_capture --all-features (4 passed)
• cargo make lint-rust
• cargo make test-rust (378 passed, 92 skipped)
• cargo make check
• Docker-backed cargo make test-rust-integration (92 passed, 378 skipped; nextest reported 1 leaky with successful exit)
• Docker-backed cargo make check-trace-gate (passed twice after starting the trace-gate Postgres fixture)

Review

• Scout selected docs/source_capture.rs as a strong low-risk modularization target.
• Skeptic found no code-level blocking issue; its only caution was its own trace-gate environment missing Postgres, which was closed by rerunning cargo make check-trace-gate successfully in the main thread.