feat(cms): U4 one-command site build + local preview

Add tools/build_site.py: runs the full pipeline in dependency order
(build_content -> build_listings -> seo_inject --all --sitemap ->
build_botlinks -> build_search_index), each as a subprocess so a failing
step aborts the build before anything downstream runs (no partial
deploy). --check verifies non-destructively; --images also runs
optimize_images. This is what CI will run in U5.

Fix a build_content idempotence bug found via U4: the hero and
article-header regions were emitted with leading indentation while the
regex left the template's pre-tag whitespace in place, so indentation
compounded (+16 spaces) on every rebuild. First line is now unindented;
the full build is verified idempotent (identical git tree across two
runs). Added a fixpoint test (re-rendering a page's own output is
byte-identical) so this can't regress.

docs/local-preview.md: build + serve on :8000 in all three languages.
tools/README.md: document build_site + build_content.

26 tests pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tools/build_content.py

@@ -118,7 +118,9 @@ def render_article_header(meta):
              f'                    <h1>{html_mod.escape(meta["title"])}</h1>']
     if meta.get("subtitle"):
         lines.append(f'                    <p class="article-subtitle">{html_mod.escape(meta["subtitle"])}</p>')
-    return "\n".join(lines)
+    # First line carries no indent: the template's pre-tag whitespace positions
+    # it. Adding our own would compound on every rebuild (non-idempotent).
+    return "\n".join(lines).lstrip()
 
 
 def render_hero(hero, css_class):
@@ -138,7 +140,9 @@ def render_hero(hero, css_class):
     if hero.get("caption_html"):
         fig.append(f'                    <figcaption>{hero["caption_html"]}</figcaption>')
     fig.append('                </figure>')
-    return "\n".join(fig)
+    # First line unindented — the template's pre-tag whitespace positions it
+    # (otherwise indentation compounds on every rebuild).
+    return "\n".join(fig).lstrip()
 
 
 # ------------------------------------------------------------- page assembly