feat(lang): new Type { field: value } record initialisers

`new` accepted only a bare `new T` (every field its declared default) or
`new []T`, but the docs (kw-new) document `new Record { field: value, … }`
as the way to construct a record with non-default fields — a documented,
intended form the parser never accepted (`let o = new Point { x: 3 }` failed
with "expected newline or ';'").

Parse an optional `{ … }` override record after the type in a `new`
expression (reusing the existing `record()` parser that `spawn` uses), and
seed each field in emit_new_struct from that record when present, else from
the field's declared default. `new []T` and bare `new T` are unchanged.

Also mark the illustrative kw-import fence `# doc-check: skip` (its imports
are example paths that can't resolve in isolation), which makes `x check-docs`
fully green (398 fences, 0 drifted) — so it is now wired as a gate in
`x test-tools` and CI, guarding against future doc/compiler drift.

Reseed is a clean fixpoint (x bootstrap-cfree holds); x test (56),
x selfhost-test (29, golden renders unchanged) and x test-tools (30) green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.forgejo/workflows/ci.yml

@@ -80,5 +80,6 @@ jobs:
           # site generator (gen.py) is still Python; its port is tracked.
           bin/x check-impl
           bin/x check-vocabulary
+          bin/x check-docs
           python3 tools/docgen/gen.py --out build/pages
           python3 tools/docgen/check.py build/pages