Add top-level fence marker for unwrapped, compile-only Scala blocks

[russwyte]

What

Adds a top-level fence modifier that compiles a Scala block verbatim as its own compilation unit — no object MarklitWrapper: def run() wrapper. Such blocks are compile-only (code + diagnostics render, never executed). A top-level definition can be hoisted into a running example via id=/extends=.

```scala marklit:top-level,id=actions
enum CounterAction:
  case Inc
  case Set(v: Int)
```

```scala marklit:extends=actions
val a: Any = CounterAction.Set(10)
a match
  case CounterAction.Set(v) => println(s"set $v")   // compiles clean, prints "set 10"
  case _                    => println("other")
```

Why

Wrapping every block in def run() makes user code a method body, which breaks or warns on top-level-only constructs (empirically confirmed against the bundled shim):

• Local-class warning (the real driver): matching a parameterized enum/ADT case against a non-local scrutinee emits the type test for X cannot be checked at runtime because it's a local class — because an enum inside def run() is a local class. This is the warning that forced show-warnings=false across the conduit docs. Hoisting the enum to file scope removes it.
• opaque type and @main def fail outright inside the wrapper.

vs. mdoc

Confirmed from mdoc's own docs: mdoc wraps blocks in a class, and its only escape (reset-object) wraps in an object and resets the scope. It has no top-level modifier and can't share a definition forward. marklit's top-level compiles at genuine file scope and hoists forward.

How

• Modifier.TopLevel + CodeBlock.isTopLevel/modifierConflicts — top-level is strict (only scope options, a version selector, and show-warnings may accompany it).
• ScopeContext carries topLevel + topLevelPriorCode; ScalaCompiler bypasses the wrapper/marker for top-level blocks and hoists inherited definitions above the wrapper for normal consumers.
• ScopeManager tags scopes by kind, splits inherited code into hoist/body buckets, and guards cross-kind extends/append/id-reuse. A normal block may extend a top-level scope; the reverse is rejected.
• Version selectors compose: top-level,scala=3.7.3 compiles against that version, and a consumer must agree.
• BlockCacheKey includes the new inputs (key prefix bumped to v3).

Docs & example

• README: feature-matrix rows (incl. the mdoc comparison) + a "Top-level blocks" section.
• New worked example examples/base/src/main/markdown/top-level.md (added to the index), demonstrating the warning contrast, opaque type, @main, and a cross-version given/extension. Verified by rendering it through the example's sbt build.

Tests

Positive / negative / pathological coverage across model, parser, compiler, cache, scope, processor, and end-to-end (MarklitSpec, both isolated and --page-scope) layers, including a regression test pinning the local-class warning behavior and cross-version cases. Full suite green (261 core + 63 compiler + Java).