fix(docs): repair EnvNamespace rustdoc link

[zackees]

Incident evidence

Validated against origin/main at 2705844b9c061b060a26e09ffda4b22c1766a4db on 2026-07-13.

The Documentation run fails with warnings denied:

error: unresolved link to `EnvNamespace`
  --> crates/fbuild-build-engine/src/pipeline/context.rs:237:27
note: -D rustdoc::broken-intra-doc-links implied by -D warnings

The method returns and constructs fbuild_core::EnvNamespace, so the intended public item is unambiguous.

Implementation runbook

1. Open crates/fbuild-build-engine/src/pipeline/context.rs.
2. Change the link target in the Build the typed ... doc comment from:

   [`EnvNamespace`]

   to:

   [`fbuild_core::EnvNamespace`]

3. Do not escape the brackets, suppress broken_intra_doc_links, or add an otherwise-unused import. The link must resolve to the real public type.
4. Check the nearby return type and constructor still refer to the same fbuild_core::EnvNamespace item.

Verification

While soldr is temporarily unavailable, use source review and the Documentation workflow; do not replace soldr with bare cargo. When soldr is healthy:

RUSTDOCFLAGS="-D warnings" soldr cargo doc -p fbuild-build-engine --no-deps
RUSTDOCFLAGS="-D warnings" soldr cargo doc --workspace --no-deps
git diff --check

On PowerShell, set $env:RUSTDOCFLAGS = "-D warnings" before the soldr command.

Acceptance criteria

• The intra-doc link resolves to the public fbuild_core::EnvNamespace definition.
• Documentation completes with warnings denied.
• No lint suppression or unrelated documentation edits are introduced.