Heddle v2.0

[multiarc]

Heddle 2.0

Forms the 2.0.0 release: the full v2 language and tooling effort, merged as a single
breaking window per the breaking-windows policy.
Merge to main, tag v2.0.0, release. Full details in the CHANGELOG;
the as-shipped window record is in docs/spec/records.md.

Highlights

• Native expression tier — a sandbox-safe, Roslyn-free expression language in call
  parens (@if(Count > 0), @(Price * Quantity)), compiled to
  System.Linq.Expressions; new TemplateOptions.ExpressionMode
  (MemberPathsOnly / Native (default) / FullCSharp).
• Safe-by-default HTML output — OutputProfile.Html encodes the unnamed @(...);
  @raw opts out, @profile flips mid-document; pluggable TemplateOptions.Encoder
  with byte-identical default; context encoders @attr / @js / @url.
• Declarative branching — @elif / @else / @ifnot via the public
  Scope.Publish/TryRead channel and [BranchRole]; custom extensions can join
  branch sets.
• Loop & directive ergonomics — @for(5) / @for(range(...)), @list,
  TrimDirectiveLines.
• Named props & slots — typed definition parameters with defaults, parameterized
  @out() slots, abstract composition and narrowing; positioned compile errors.
• Streaming & UTF-8 sinks — TextWriter and IBufferWriter<byte> render overloads
  with O(1)-allocation transcoding; render budgets for untrusted templates.
• Build-time precompilation — Heddle.Generator compiles .heddle templates at
  build time, byte-identical to the dynamic engine; positioned HED70xx build
  diagnostics; version-locked with the engine.
• Tooling — stable HEDxxxx diagnostic IDs, LSP server + VS Code extension, v2 Ace
  grammar with in-browser demo, the heddle CLI (T4 successor), ten golden-asserted
  samples doubling as CI coverage.

Breaking changes (the 2.0 window)

• OutputProfile default: Text → Html.
• TrimDirectiveLines default: false → true.
• Generator MSBuild defaults follow the engine (Html / true).
• Heddle.Models.ForModel deleted → immutable Heddle.Models.Range struct; only
  rendered-byte effect is range stringification.
• Legacy @import() removed — positioned HED4003 error naming @<< / @partial
  replacements.
• AllowCSharp is [Obsolete] (still functional) — use ExpressionMode.
• 1.x precompiled assemblies are rejected by the engine-version gate and fall back per
  PrecompiledMismatchPolicy; rebuild with the 2.0 generator.