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/content/docs/integrations/agent-clients.mdx

@@ -285,7 +285,7 @@ Admin CLI skills call the same KTX CLI commands:
 | `ktx sl list --json` | List semantic-layer sources |
 | `ktx sl search <query> --json` | Search semantic-layer sources |
 | `ktx sl validate <source> --connection-id <id>` | Validate semantic source definitions |
-| `ktx sl query --format json` | Execute a semantic-layer query when semantic compute is configured |
+| `ktx sl query --format json` | Execute a Semantic Query when semantic compute is configured |
 
 ### Security constraints