feat(typediagram): add discriminants, untagged unions, and target gating

[abdushakoor12]

TLDR

Add three typeDiagram language features end-to-end: explicit numeric union discriminants, untagged union syntax, and declaration-level target gating with @targets(...) / @skipTargets(...), and teach converters/rendering/docs to honor them. Closes #25, closes #26, closes #27.

Details

Added:

• Explicit numeric discriminants on union variants such as ParseError = -32700
• untagged union parsing/modeling/printing, with Rust #[serde(untagged)] emission and plain TypeScript union emission
• Declaration-level target gating annotations with @targets(...) and @skipTargets(...)
• Shared variant helpers in packages/typediagram/src/variant.ts
• Target-visibility helpers in the model layer so all emitters filter declarations consistently

Changed:

• Parser, AST, model builder, JSON round-trip, pretty-printer, layout text, and SVG rendering now preserve discriminants, untagged unions, and declaration targeting metadata
• All language emitters now filter declarations via shared target gating instead of emitting every declaration unconditionally
• Core docs and README now document explicit discriminants, and the VS Code sample includes a discriminant example
• Bundle-size budget for typediagram-core increased to 80 KB to account for the added parser/emitter surface

Deleted:

• A few now-unused imports and no-op tree-shake suppression lines that became unnecessary after the refactor

Spec/Doc changes:

• Updated docs/specs/language-reference.md grammar and examples for explicit numeric discriminants
• Updated packages/typediagram/README.md with discriminant examples

Breaking changes:

• No breaking API change intended; this is additive syntax and emitter behavior.

How Do The Automated Tests Prove It Works?

• packages/typediagram/test/converters/rust.test.ts
  • preserves explicit numeric discriminants on enum variants
  • emits serde untagged enums from untagged union syntax
• packages/typediagram/test/converters/typescript.test.ts
  • emits untagged tuple unions as plain TypeScript unions
  • emits remaining untagged payload shapes without discriminator fields
  • [CONV-TS-BUG-27] skips declarations gated away from the typescript target
  • [CONV-TS-BUG-27] supports blacklisting the typescript target
• packages/typediagram/test/parser.test.ts
  • parses untagged unions
  • parses target annotations on declarations
  • malformed/unknown annotation recovery coverage
• packages/typediagram/test/model.test.ts
  • preserves untagged unions through printSource and JSON
  • preserves declaration target gating through printSource and JSON
  • filters declarations for a specific target
• packages/typediagram/test/render.test.ts
  • renders explicit union discriminants in the SVG text
• make ci passed locally, including full Playwright coverage merge and the final bundle-size check (79.38 KB within the 80 KB budget)