Phase 2: unified query language spanning ClickHouse and Tantivy

Replaces the separate SQL-only /query and text-only /search endpoints
with one pipe-syntax query language (plus raw SQL escape hatch) that
compiles to a single IR and execution plan across both backends, so a
query like `message:"connection refused" | stats count by host` runs
as one request instead of two disjoint tools.

- api/internal/querylang: lexer -> ast -> parser -> ir -> planner ->
  executor, each layer independently tested.
- Execution generalizes Phase 1's proven Tantivy-prefilter pattern
  into a 4-way routing table (pure ClickHouse / text-only / text +
  aggregation / raw SQL passthrough).
- Unified web query page and `sentryctl query`, both hitting the same
  POST /query endpoint.
- Benchmarked against a real 1,022,000-row dataset
  (hack/benchmark-fixture); caught and fixed a real bug where the
  Tantivy prefilter cap (10,000) produced an IN-clause exceeding
  ClickHouse's default max_query_size -- lowered to 5,000, documented
  in docs/query-language-design.md and docs/phase-2-runbook.md.
- docs/query-language-reference.md: customer-facing syntax reference.

web/src/routes/+layout.svelte

@@ -1,6 +1,5 @@
 <script lang="ts">
 	import favicon from '$lib/assets/favicon.svg';
-	import { page } from '$app/state';
 
 	let { children } = $props();
 </script>
@@ -10,32 +9,4 @@
 	<link rel="icon" href={favicon} />
 </svelte:head>
 
-<nav>
-	<a href="/" class:active={page.url.pathname === '/'}>SQL Query</a>
-	<a href="/search" class:active={page.url.pathname === '/search'}>Full-Text Search</a>
-</nav>
-
 {@render children()}
-
-<style>
-	nav {
-		font-family: system-ui, sans-serif;
-		max-width: 960px;
-		margin: 1rem auto 0;
-		padding: 0 1rem;
-		display: flex;
-		gap: 1rem;
-		border-bottom: 1px solid #ccc;
-	}
-	nav a {
-		padding: 0.5rem 0;
-		text-decoration: none;
-		color: #555;
-		border-bottom: 2px solid transparent;
-	}
-	nav a.active {
-		color: #000;
-		border-bottom-color: #000;
-		font-weight: 600;
-	}
-</style>