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/tests/docs-index-route.test.mjs

@@ -112,6 +112,18 @@ test("/ktx/docs redirects to the docs introduction", async () => {
   );
 });
 
+test("/ redirects into the /ktx docs site", async () => {
+  const response = await fetch(`${docsSiteUrl}/`, {
+    redirect: "manual",
+  });
+
+  assert.equal(response.status, 307);
+  assert.equal(
+    response.headers.get("location"),
+    `${docsBasePath}/docs/getting-started/introduction`,
+  );
+});
+
 test("/ktx/api/search returns docs search results", async () => {
   const response = await fetch(
     `${docsSiteUrl}${docsBasePath}/api/search?query=setup`,

docs-site/tests/docs-search-behavior.test.mjs

@@ -1,5 +1,5 @@
 import assert from "node:assert/strict";
-import { readFile } from "node:fs/promises";
+import { access, readFile } from "node:fs/promises";
 import { dirname, join } from "node:path";
 import { test } from "node:test";
 import { fileURLToPath } from "node:url";
@@ -17,6 +17,23 @@ test("root provider uses the base-path-aware search API", async () => {
   assert.match(layout, /api:\s*"\/ktx\/api\/search"/);
 });
 
+test("metadata icons include the docs base path", async () => {
+  const layout = await readDocsFile("app/layout.tsx");
+
+  assert.match(layout, /icon:\s*"\/ktx\/brand\/ktx-mascot\.svg"/);
+  assert.match(layout, /shortcut:\s*"\/ktx\/brand\/ktx-mascot\.svg"/);
+  assert.doesNotMatch(layout, /:\s*"\/brand\/ktx-mascot\.svg"/);
+});
+
+test("markdown negotiation uses the Next proxy convention", async () => {
+  await assert.doesNotReject(access(join(docsSiteDir, "proxy.ts")));
+  await assert.rejects(access(join(docsSiteDir, "middleware.ts")));
+
+  const proxy = await readDocsFile("proxy.ts");
+  assert.match(proxy, /export function proxy/);
+  assert.doesNotMatch(proxy, /export function middleware/);
+});
+
 test("site background stacking does not target every body child", async () => {
   const css = await readDocsFile("app/global.css");