Add scope and preprocessor-directive helpers for common code-gen shapes

[matt-edmondson]

Problem

CodeBlocker ships exactly two scopes — Scope ({ … }) and ScopeWithTrailingSemicolon ({ … };). Every real generator built on it re-implements the same handful of shapes on top of raw WriteLine.

From ktsu.Semantics' LogarithmicScalesGenerator alone:

cb.WriteLine("#nullable enable");
cb.NewLine();
cb.WriteLine("namespace ktsu.Semantics.Quantities;");
cb.NewLine();
cb.WriteLine("using System;");
cb.WriteLine("using System.Globalization;");
cb.WriteLine("using System.Numerics;");
cb.NewLine();

None of that is domain-specific, and hand-writing it means every generator gets to invent its own spacing conventions.

Proposal

Add the shapes that recur in generated C#. Suggested surface, all opt-in:

Scopes

• ParenScope / BracketScope — ( … ) and [ … ], for long argument lists and collection expressions.
• IndentScope — indent without braces, for continuation lines (generic constraints, chained calls).
• RegionScope — #region name … #endregion.
• DirectiveScope — #if COND … #endif.
• PragmaScope — #pragma warning disable CSxxxx … #pragma warning restore CSxxxx, which is easy to get wrong by hand and leaks if unbalanced.

Directive and preamble helpers

• WriteNullableEnable() / WriteNullableDisable().
• WriteAutoGeneratedHeader(string? copyright = null) — the // <auto-generated /> preamble every generator emits.
• WriteFileScopedNamespace(string) and WriteUsings(IEnumerable<string>), with consistent blank-line handling.

Each scope should follow the existing ktsu.ScopedAction pattern so it composes with using the same way Scope does, and should respect the configured indent string and newline (#81).

Notes

• Keep the additions purely additive — no change to Scope / ScopeWithTrailingSemicolon.
• These are C#-flavoured but harmless for other curly-brace languages; if that becomes a problem, a later issue can split language-specific helpers into their own namespace.
• Balance is the point: an unbalanced #pragma warning disable or #region in generated source is a compile-time or IDE-level annoyance that is tedious to trace back.

Acceptance criteria

• Each new scope emits its opening and closing text and restores the indent level on dispose, including when the body throws.
• Nested scopes of mixed kinds indent correctly.
• README.md gains a short example per new scope.

Context

Part of ktsu-dev/Semantics#181 — generalizing the code-generation stack so more projects can use it.