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/app/layout.tsx

@@ -28,8 +28,8 @@ export const metadata: Metadata = {
   description:
     "Open-source context infrastructure that makes agentic analytics reliable.",
   icons: {
-    icon: "/brand/ktx-mascot.svg",
-    shortcut: "/brand/ktx-mascot.svg",
+    icon: "/ktx/brand/ktx-mascot.svg",
+    shortcut: "/ktx/brand/ktx-mascot.svg",
   },
 };