test: fix lint include checks

[nexicturbo]

Bounty submission for #32.\n\nScope:\n- Make test/lint/lint-includes.py pass on the current tree.\n- Remove duplicate includes from src/span.h.\n- Drop boost/process.hpp from EXPECTED_BOOST_INCLUDES because it is no longer used.\n- Convert the include lines reported by lint-includes.py from quote syntax to bracket syntax.\n\nVerification:\n- python test\lint\lint-includes.py\n- git diff --check\n\nPayment details are available on request after maintainer review/approval.

[nexicturbo]

Quick status refresh for bounty review.

This PR remains open, non-draft, and mergeable. Scope is limited to making test/lint/lint-includes.py pass on the current Bitgesell tree by:

• removing duplicate src/span.h includes
• removing obsolete boost/process.hpp from the expected Boost include list
• converting the lint-reported include lines from quote syntax to bracket syntax

Validation previously run:

• python test\lint\lint-includes.py
• git diff --check

No contributor-side revision is pending unless maintainers want a focused adjustment.