Feature: Site Build

Purpose

The project contains a static-site input directory (site.in/) and a generated static-site output directory (site.out/).

The Site Build feature provides the script that converts site.in/ into site.out/.

scripts/site-build.sh must generate the static site in site.out/ from the input in site.in/.
Each site.in/*.txt input file must produce site.out/<name>.html.
Every non-.txt, non-template.html file under site.in/ (authored assets such as scripts, styles, and images) must be copied verbatim into site.out/.
Pages may contain __KEY__ placeholders for live state; values come from the state file (dataflow.out/site-state.txt by default, overridable with SITE_STATE_FILE), written by scripts/site-state-fetch.sh. When the state file is absent, remaining placeholders render as “unavailable”, so the build stays portable.
Generated output must be identified as generated.
The script must accept optional input and output directory arguments.
When no arguments are given, the script must use site.in/ and site.out/ relative to the repository root.
The script must be a POSIX shell script.
The HTML page structure must live in site.in/template.html, not in the build script.
site.in/template.html must contain the marker line  where page content is inserted.