Wrap higher-order predicate errors with source text

[myzie]

Summary

Predicate errors raised inside map/filter/any/all/find/count are now wrapped with the form name, the predicate's source text (printed back from the AST), and the failing element's index.

map predicate `it.Nmae` failed on element 0: expr: evaluate error: key "Nmae" not found (did you mean "Name"?)

Nested forms add their own layer, top-down through the iteration tree. The internal __expr_map__ rewrite is reversed before printing, so users see the source they typed:

map predicate `map(it, it.bad)` failed on element 0: map predicate `it.bad` failed on element 0: ...

Behavior details

• Trapped: anything wrapping ErrEvaluate. Wrapping preserves the chain, so errors.Is(err, ErrEvaluate) still matches.
• Not wrapped: raw context.Canceled / context.DeadlineExceeded so cancellation stays observable.
• Backtick guard: predicates whose printed source contains a backtick fall back to a position-only form (map predicate failed on element 0: ...), since backticks are the surrounding delimiters.
• Fast-path parity: tryFilterIndex in program.go (the filter(xs, p)[N] fast path) uses the same helper, so the fast path is not a debuggability regression.

Implementation

• New wrapPredicateErr(name, predicate, index, err) in higher_order.go.
• New formatPredicate(node) uses go/printer.Fprint with a fresh FileSet and post-processes the map sentinel back to map. No new external deps; go/printer is stdlib.
• forEach now takes a form name and the predicate AST. Each of the six forms passes its own user-visible name.

Tests

higher_order_test.go gained six new tests:

• TestHigherOrder_PredicateError (updated): pins the new format on a map(...) typo.
• TestHigherOrder_PredicateErrorReportsIndex: failure on the 3rd element reports element 2.
• TestHigherOrder_PredicateErrorNestedForms: both layers appear; sentinel does not leak.
• TestHigherOrder_PredicateErrorFormNames: spot-check filter / find / count / any / all.
• TestHigherOrder_PredicateErrorFilterIndexFastPath: fast path uses the same wrapping.
• TestHigherOrder_PredicateErrorBacktickFallback: predicate with a backtick falls back cleanly.
• TestHigherOrder_PredicateErrorIgnoresCancellation: cancellation is not wrapped.

Test plan

• go test ./...
• go test -race ./...
• FuzzCompile 20s
• FuzzEval 20s