[cscheid]

Stacked on #949 (base: feature/lua-api-q1-parity). Strand bd-irkfpip1, a follow-up noted in #949's plan.

What changes

The {{< env NAME >}} shortcode now reads the same per-render environment snapshot that #949 introduced for Lua filters' and shortcode scripts' os.getenv (StageContext::env_snapshot: real environment, then _environment file values for missing keys, then QUARTO_PROFILE forced to the active profile list). Before this, EnvShortcodeHandler re-did that three-way merge at every lookup with std::env::var on a worker thread, so the shortcode and a filter could in principle disagree, and the real environment was read in two places.

• ShortcodeResolveTransform::with_lua_support and the two pipeline builders (build_transform_pipeline, build_q2_preview_transform_pipeline) take an Option<Arc<EnvSnapshot>> in place of the project_env + quarto_profile pair. The transform hands the same Arc to the env handler and to the Lua engine's HostContext.
• That made the RenderContext.env_snapshot bridge added in Lua API: quarto.project, quarto.doc paths, comparable quarto.version, os.exit fails the document (GH #939-#942) #949 unnecessary; it is removed.
• Without a snapshot (custom pipelines, unit tests that build a transform directly) the handler reads the real process environment, exactly as before.
• All 47 builder call sites had the same trailing Default::default(), None shape and were rewritten by script; no behavior change at any of them.
• The handler's tests now drive a snapshot directly instead of mutating the process environment, except the one that pins the no-snapshot fallback.

Verification

• cargo clippy -p quarto-core --all-targets -- -D warnings clean; cargo nextest run -p quarto-core -p quarto green (includes the contract-env smoke fixture and the lua_host_context_cli e2e test, whose shortcode half exercises the transform-held snapshot).
• claude-notes render gate green for the plan update.
• Nothing platform-specific; Windows cross-check has the same C-dependency blocker as Lua API: quarto.project, quarto.doc paths, comparable quarto.version, os.exit fails the document (GH #939-#942) #949.

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