lsp: hit-test coverage gaps — end-of-identifier caret, use directives, fn type params, enum variant declarations, function-local consts

[0xGeorgii]

Follow-up from the comprehensive review of PR #239 (LSP server). Line numbers reference the PR branch at review time.

Severity: major (first item), minor (rest) — hit-testing coverage gaps in ide/ide-db, all empirically confirmed with the public Analysis API.

1. Caret at the exclusive end of an identifier misses (goto_definition.rs:37, hit_test.rs): hit_test covers start <= offset < end, and goto/hover pass the raw LSP offset with no fallback, so F12 with the caret immediately after a name (where double-click/typing leaves it) returns "no definition found". On return target();: goto at first byte → Some, last byte → Some, exclusive end → None. completions.rs:285-301 already implements the identifier-biased one-byte fallback (enclosing_hit); goto/hover do not share it. Fix: extract the fallback into ide-db next to hit_test and use it from goto_definition and hover.
2. use directives are not hit-testable (hit_test.rs:42): the walk starts exclusively from arena[file].defs and never considers SourceFileData::directives, so goto/hover anywhere on use lib::helper; returns None. Directive segments and braced imports are arena-backed IdentIds with real locations (nodes.rs:283-284) — the data exists, it is simply never walked.
3. Def::Function::type_params omitted from def_children (hit_test.rs:107): hover on the declared T' in pub fn id T'(x: T) -> T returns None; the parser lowers type params with real locations (lower.rs:374-382) but the Def::Function arm destructures with ...
4. Enum variant declarations unreachable (goto_definition.rs:355): goto/hover on the declared variant Red in enum Color { Red, Green } return None; goto_in_def covers only Function/ExternFunction args and Struct fields, and hover's ident_in_def has the same gap — unlike every other declaration name (goto_definition_name_reaches_itself).
5. Function-local consts invisible (symbols.rs:29): collect_def recurses only into Struct methods and Spec defs, never into function bodies, so a Def::Constant reached via Stmt::ConstDef is not in file_defs, and neither resolve_local/in_scope_locals (VarDef-only) nor find_def_* reach it — goto on a local-const reference silently returns None, despite the doc claiming it "Collects every definition in file".