Make the text DSL a first-class, runnable graph description

[psyinf]

Summary

Until now the text DSL could only be parsed, validated and drawn; running a runtime graph still required JSON. This PR makes the DSL executable and the primary way to describe runtime graphs. The JSON format stays fully supported.

Changes

Running DSL graphs

• DslFilterGraph<In, Out> (DslFilterGraph.hpp) builds, type-checks and runs named-edge DSL graphs, and is itself a MessageFilter, so it can be nested or registered as a stage.
  • Stages run in source order, waiting until their inputs have been produced.
  • Every reader of an edge gets its own copy (fan-out).
  • A dropped message (std::nullopt) skips everything downstream.
  • Merges receive an empty slot ("hole") for each dropped edge.
  • Values routed to end are discarded.
• The output type sets the graph's output shape: a concrete type (exactly one out), GraphOutputs (several, optionally keyed outputs of different types) or Void (sink-only).
• MergeFilter / registerMergeFilter provide fan-in stages for (a, b) -> Merge, using the same combiner signature as JoinFilter.
• Construction checks the whole graph up front: unknown stages, bad arguments, type mismatches along every edge, fan-in and Void misuse, and the output shape. All problems are thrown together in one GraphError with line:column; validateDslGraph<In, Out>(text) returns them without throwing.

Parser

• dsl::parseGraphProgram now uses the lexy-based parser, which moved into GraphLang.hpp.
• Syntax errors are located and descriptive, e.g. 1:17: expected '->' but found 'Print'. Previously the lexy path reported every problem as "syntax error" at column 1.
• A syntax error ends its statement, so there are no follow-on errors.
• New checks: malformed or out-of-range numbers, empty groups (), and unterminated strings, which no longer swallow the following lines.
• Added support for decimal numbers, string escapes and # inside strings.
• Deprecated: the hand-written parser moved to GraphLangHandwritten.hpp as [[deprecated]] dsl::parseGraphProgramHandwritten. Tests check it against the lexy parser for identical output and diagnostics. parseGraphProgramLexy remains as an alias.
• A stage written without arguments now receives {} instead of null, matching JSON stages without "config".
• toMermaid uses generated node ids (no clashes with end or internal $out edges), labels outputs and lists stage arguments.

Docs and example

• README, EXAMPLE.md and apps/textPipeline lead with the DSL: tap, merge, named outputs, diagnostics, and the same pipeline in JSON.
• Corrected two errors in EXAMPLE.md: the main-path output was shown in uppercase, and the AB branch line was missing.
• README now lists lexy as a dependency.
• CHANGELOG has an Unreleased entry.

Testing

• Windows / MSVC (VS 18), windows-msvc-debug-developer-mode preset (ASan, /W4): 55/55 tests pass, no compiler warnings; the textPipeline output matches EXAMPLE.md.
• New tests cover execution semantics, GraphOutputs, Void graphs, nesting, every build-time check, and parser diagnostics: 20 malformed inputs with exact expected messages, run through both parsers.
• The README code snippets were compiled and run separately.
• Not built locally with GCC or Clang; CI covers linux-gcc.

Notes

• GraphLang.hpp (and therefore DslFilterGraph.hpp) now includes lexy. Consumers of the CMake target are unaffected, since lexy is an external include there. Projects adding lexy's headers as a regular include path at /W4 will see MSVC warning C4310 from inside lexy.
• Known limitation: stage arguments are flat key=value pairs; nested objects and lists are not expressible yet.