ERR/E0.2: raise / try / catch / onfail + precedence + consumer-aware pipe (parser)

Parser-only second step of the error-handling stream. No sema/codegen.

- token: 4 keywords — `raise`, `try`, `catch`, `onfail`.
- ast: RaiseStmt, TryExpr, CatchExpr {operand, binding?, body, is_match_body},
  OnFailStmt {binding?, body}.
- parser:
  - `try` is a unary prefix (binds tighter than `or`; right-recursive so it
    stacks under `xx`/`@`/etc).
  - `or` is already left-associative (precedence-climbing loop) — no change.
  - `catch` is a postfix with four body shapes (no-binding block / block /
    bare-expr / `== { case }` match-body, the latter reusing parseMatchBody
    with the binding as subject).
  - `raise EXPR;` and `onfail [e] { } | onfail EXPR;` statements; `error`
    parses in expression position so `raise error.X` works; raise rejected
    in expression position and inside an onfail body (in_onfail_body flag).
  - consumer-aware `|>`: pipes the LHS into the head call through a
    try/catch/or wrapper, preserving the wrapper.
- print: printExpr + match-arm printing for round-trips (anyerror!void to
  break the printExpr<->printMatchArms inferred-error-set loop).
- sema/lsp: exhaustive switch arms for the 4 nodes + 4 keyword tokens.
- tests: ~22 inline parser tests (precedence, all catch forms, both
  rejections, pipe cases, round-trip prints incl. match-body).

zig build, zig build test (264), and 254/254 examples green.

src/lsp/server.zig

@@ -1653,6 +1653,10 @@ pub const Server = struct {
             .kw_false,
             .kw_enum,
             .kw_error,
+            .kw_raise,
+            .kw_try,
+            .kw_catch,
+            .kw_onfail,
             .kw_case,
             .kw_break,
             .kw_continue,