Add Dart + Protobuf converters; make all 8 converters lossless round-trip

[MelbourneDeveloper]

Closes #2 (protobuf support).

TLDR

Adds Dart and Protobuf converters, rewrites C#/Go/Python/Rust to round-trip HOME_PAGE_SAMPLE byte-for-byte, and wires the new languages into the web converter page.

Details

New converters

• Dart (packages/typediagram/src/converters/dart.ts): sealed-class DUs (sealed class + final class ... extends), typedef aliases, Option<T> ↔ T?, first-class generics.
• Protobuf / proto3 (packages/typediagram/src/converters/protobuf.ts): message for records, enum for unit-only unions, oneof + nested message for struct-variant unions, repeated/optional/map<K,V> field labels. Uses // @td-generics:, // @td-type:, // @td-alias: comment directives to encode typeDiagram concepts proto3 can't express natively (generics, Option<List<T>>, aliases).

New shared helper

• packages/typediagram/src/converters/brace-lang.ts: extractBalancedBlock, splitTopLevelCommas, formatGenericsDecl, extractTrailingNullable, parseGenericParamList. Used by C#, Dart, Protobuf.

Converter rewrites for lossless round-trip

• C#: closed-hierarchy DUs (abstract record + nested sealed records, RestClient.Net / Outcome style), using Alias = Target; aliases, primary-constructor records preserving original field names. Replaces the prior JsonPropertyName-discriminated interface pattern.
• Go: interface + marker-method (is<Union>()) encoding with union-qualified variant struct names, Go 1.18 [T any] generics, field names kept verbatim. Legacy embedded-type ifaces still parse as a fallback.
• Python (dataclass style): struct-variant unions emit as per-variant @dataclass classes plus a Name = VarA | "VarB" alias line; parser reads that line back and collapses the variant classes into union variants. Generics via Generic[T] / TypeVar. Top-level Email = str now parses as alias.
• Rust: replaced non-recursive [^}]* enum regex with a brace-balanced body extractor; added splitRsFieldList that respects angle/brace/paren depth so HashMap<String, String> survives as a single field.
• TypeScript: Option<T> emits as T | undefined; parser accepts T | undefined, T | null, T | undefined | null, T | null | undefined as Option<T>. Alias-vs-union detection excludes the option shape.
• F#: unchanged (was already lossless).

Shared test helper

• expectLosslessRoundTrip(converter) in test/converters/helpers.ts asserts TD → lang → TD is byte-for-byte identical to HOME_PAGE_SAMPLE (now exported from packages/typediagram/src/sample.ts). Each per-language test file now calls it.

Web converter page

• SupportedLang widened to include dart and protobuf.
• New tabs in converter.ts; new highlighter rule sets in converter-highlight.ts.
• playground.ts and converter.ts both source the sample from HOME_PAGE_SAMPLE — single source of truth across the app.

Makefile

• make clean now also removes per-package coverage/ dirs.
• New make clean-start target: kills anything on the Vite dev port (default 5173), full clean, build, then dev.

Budgets / config

• packages/typediagram/scripts/bundle-size.mjs: budget raised 50 → 75 KB to accommodate two new converters (~8-10 KB each, unminified parser + emitter). Current bundle: 67.81 KB.
• coverage-thresholds.json: typediagram branches 90.76 → 90.89, functions 98.5 → 98.58 (ratcheted up only).
• packages/cli/test/roundtrip.e2e.test.ts: updated expected C# output from public record to public sealed record to match the new DU emission.

How Do The Automated Tests Prove It Works?

Round-trip (all 8 languages pass):

• [CONV-FS-RT], [CONV-TS-RT], [CONV-RUST-RT], [CONV-CS-RT], [CONV-PY-RT], [CONV-GO-RT], [CONV-DART-RT], [CONV-PROTO-RT] — each calls expectLosslessRoundTrip(converter) against HOME_PAGE_SAMPLE and asserts printSource(fromSource(toSource(parsed))) === HOME_PAGE_SAMPLE byte-for-byte.

Feature coverage for new converters:

• [CONV-DART-FROM] / [CONV-DART-TO] / [CONV-DART-EDGE] / [CONV-DART-ERR] (13 tests) — verifies Dart sealed-class DU parsing, generic sealed classes with extending variants, implements as well as extends on variants, T? ↔ Option<T> field mapping, empty enums, malformed-field resilience.
• [CONV-PROTO-FROM] / [CONV-PROTO-TO] / [CONV-PROTO-EDGE] / [CONV-PROTO-ERR] (12 tests) — verifies oneof + nested message parsing, UNSPECIFIED sentinel handling, // @td-type / // @td-generics / // @td-alias directive handling, map<K, V> and Option<Map<K,V>> edge cases, malformed-line skipping.
• [CONV-BRACE-LANG] (16 tests) — covers the shared helper: balanced-brace extraction for malformed and nested inputs, angle-aware comma splitting, generic-params parsing for plain / extends / Go-style T any forms, nullable extraction, generics formatting.

Regression-guarded behaviour changes:

• [CONV-CS-BUG-10] reworded to assert the new abstract record + nested sealed record emission (no more JsonPropertyName("kind") / interface IContentItem).
• [CONV-CS-BUG-11] reworded to assert primary-constructor records preserving original field names (no more PascalCase + JsonPropertyName property-bag).
• [CONV-CS-BUG-12] reworded to assert aliases emit as using X = Y; (previously inlined).
• [CONV-GO-TO-COMPLEX] and [CONV-GO-RT] updated to assert lowercase field names and union-qualified variant struct names (e.g. ContentItemText not Text).
• [WEB-CONVERTER] renders a tab for every supported language — loosened from hardcoded 6 to "expect Dart and Protobuf tabs alongside the original 6".

Suite totals: 278 typediagram tests + 211 web tests + 59 CLI tests all pass locally under make ci. Bundle size 67.81 KB (under 75 KB budget). Coverage meets the ratcheted thresholds (lines ≥ 95%, branches ≥ 90.89%, functions ≥ 98.58% for typediagram-core).