v2 grammar follow-up: promote data decls to Decl::Data variant

[hyperpolymath]

Context

The pest grammar has a data_decl rule (line 62 of ephapax.pest); the core parser's parse_declaration routes Rule::data_decl through parse_type_decl (lib.rs:146) and the core Decl enum has no Data variant — data decls are folded into Decl::Type with the constructors encoded as a binary-sum chain. The surface AST already models data properly (SurfaceDecl::Data(DataDecl)); only the core AST is the kludge.

What to do

1. Add Decl::Data { name: Var, type_params: Vec<SmolStr>, constructors: Vec<ConstructorDef> } (or similar) to ephapax-syntax.
2. Update parse_declaration in the core parser to build the new variant instead of routing through parse_type_decl.
3. Update the desugar pass: SurfaceDecl::Data already lowers to data-registry entries (no core Decl emitted); after this change the desugar can optionally emit a real Decl::Data for downstream consumers that want the structured form.
4. Patch every exhaustive match on Decl to handle the new arm (same audit pattern used for Decl::Extern in feat(parser,desugar): extern abi blocks (clean — replaces #47/#53) #54).

Acceptance

• Decl::Data variant exists with constructor list intact
• Core parser produces the new variant for data decls
• All existing match sites compile and the workspace tests stay green
• Cosmetic only — the runtime behaviour should be unchanged

References

• Spun out from Implement v2 grammar (module/import/extern/data/match/let!) in Rust + Idris2 parsers #43 v2-grammar completion (step 5 in the re-scoped ordering).
• src/ephapax-parser/src/lib.rs parse_declaration, line 137.
• src/ephapax-syntax/src/lib.rs Decl enum, line 552.