Milestone m5: how-it-works page + Matthew Heberger credit

[CooperBigFoot]

Integration PR for milestone m5 of the pyshed-release-and-docs-modernization campaign.

• docs/how-it-works.md: plain-language explanation of the hybrid vector+raster delineation method for hydrologists, every domain term defined, mapped onto shed's staged pipeline.
• docs/credits.md: full credit to Matthew Heberger — 2025 paper (SSRN preprint DOI 10.2139/ssrn.5939056), @software{delineator} BibTeX, ORCID 0000-0001-9122-0030, Global Watersheds app, MIT note, fabric-agnostic-generalization framing, and the Djokic & Ye (1999) lineage line.

Covers vision items 13-16 (criterion 4 how-it-works/credit portions). One squashed step commit (#17).