docs: rewrite Semantic Querying concept with imperative-vs-declarative diagram

Reframe semantic-layer-internals.mdx around the contract the semantic
layer offers an agent: declare what you want (a Semantic Query), KTX
figures out how to compute it. Replaces the old "Context-Aware SQL"
framing with a clear imperative-vs-declarative narrative.

Adds a React Flow component (semantic-layer-flow.tsx) that contrasts a
buggy 4-table agent-authored SQL (chasm trap, LEFT-JOIN-in-WHERE,
hardcoded DATE_TRUNC) against the chasm-safe per-fact CTE SQL the
planner actually emits, including the outer GROUP BY over the requested
dimensions. Both lanes converge into a shared warehouse node and each
SQL card now has parallel bullet notes (failures on the left, KTX
behavior on the right).

Side fixes bundled in:
- include the /ktx basePath in the favicon metadata so the icon resolves
  under the production prefix
- migrate docs-site/middleware.ts to docs-site/proxy.ts (Next 16 rename)
- redirect / to /ktx/docs/getting-started/introduction so the apex docs
  URL works
- add tests covering the apex redirect, the favicon basePath, and the
  middleware-to-proxy rename
- propagate the Semantic Query terminology across the ktx-sl CLI
  reference, the context-layer concept page, and the agent-clients /
  primary-sources integration pages

docs-site/proxy.ts

@@ -0,0 +1,52 @@
+import { NextResponse, type NextRequest } from "next/server";
+
+const markdownMimeTypes = new Set([
+  "text/markdown",
+  "text/x-markdown",
+  "application/markdown",
+]);
+
+export function proxy(request: NextRequest) {
+  if (!isMarkdownPreferred(request.headers.get("accept"))) {
+    return NextResponse.next();
+  }
+
+  const { pathname } = request.nextUrl;
+  const docsIndex = pathname.indexOf("/docs/");
+  if (docsIndex < 0 || pathname.endsWith(".md")) {
+    return NextResponse.next();
+  }
+
+  const rewriteUrl = request.nextUrl.clone();
+  rewriteUrl.pathname = `${pathname.slice(0, docsIndex)}/llms.mdx${pathname.slice(docsIndex)}`;
+
+  return NextResponse.rewrite(rewriteUrl);
+}
+
+export const config = {
+  matcher: ["/docs/:path*"],
+};
+
+function isMarkdownPreferred(acceptHeader: string | null) {
+  if (!acceptHeader) return false;
+
+  const accepted = acceptHeader
+    .split(",")
+    .map((entry, index) => {
+      const [type = "", ...parameters] = entry.trim().split(";");
+      const quality = parameters
+        .map((parameter) => parameter.trim())
+        .find((parameter) => parameter.startsWith("q="));
+
+      return {
+        type: type.trim().toLowerCase(),
+        quality: quality ? Number.parseFloat(quality.slice(2)) : 1,
+        index,
+      };
+    })
+    .filter((entry) => Number.isFinite(entry.quality) && entry.quality > 0)
+    .sort((a, b) => b.quality - a.quality || a.index - b.index);
+
+  const preferred = accepted[0]?.type;
+  return preferred ? markdownMimeTypes.has(preferred) : false;
+}