lsp: expose extern items + data decls as navigable symbols #75

Background

ephapax-lsp::collect_decl_symbols (or equivalent — src/ephapax-lsp/src/main.rs around lines 540–550) returns None for two Decl variants when building the symbol list:

• Decl::Extern { .. } => None — TODO comment references the now-closed Implement v2 grammar (module/import/extern/data/match/let!) in Rust + Idris2 parsers #43 phase 2B work, but extern items are otherwise wired up: the typechecker registers their signatures, the wasm backend emits (import ...) directives, and tests cover both. Only the LSP symbol path is missing.
• Decl::Data { .. } => None — TODO comment references a "v2 grammar follow-up: promote data decls to Decl::Data variant #60 follow-up" but no successor issue exists. Surface-pipeline data decls reach the LSP via the desugar registry, but the core-parser path that constructs Decl::Data directly is not indexed.

Both gaps mean hover, go-to-definition, and outline don't surface extern functions or data-type constructors in editors using the compiler-integrated LSP.

What's needed

1. Add LSP symbols for every ExternItem::Fn { name, params, ret, .. } in Decl::Extern { abi, items }. Symbol kind Function, with signature showing the declared params → ret, plus the abi as part of the documentation/detail.
2. Add an LSP symbol for Decl::Data { name, type_params, constructors } (kind: Class or Enum), and one nested symbol per ConstructorDef (kind: EnumMember).
3. Update the surrounding DeclKind enum (or whatever drives the symbol kind dispatch) to carry these.

Acceptance

• gh code-search-equivalent: hovering over Some in a test file shows the Option(a) data type + ctor signature.
• Go-to-definition on a call to an extern fn jumps to the extern block declaration site.
• Document outline (textDocument/documentSymbol) lists every extern fn and data ctor.

Notes

• The standalone heuristic LSP at tools/ephapax-lsp/ has its own TODOs for hover/definition/symbols — those are intentional stubs (documented in the module header) and out of scope here.
• Also clean up the stale TODO(ephapax#43 phase 2B) comments at the same time. The wasm/typing comments are already obsolete (the work landed in feat(wasm): emit extern fn imports + direct calls (clean v2 — replaces #56) #58); just update or delete them as part of this PR.