fix: require the trace proof term to start on the line of the closing brace

[hwbehrens]

set_option ... in fails to parse when the immediately preceding command is a trace command:

sat trace [can_acquire] {
  acquire
}

set_option veil.violationIsError false in
sat trace [handoff] {
  acquire
  release
  acquire
}

error: unexpected token 'sat'; expected spec
info: ✅ Satisfying trace found

The trace command's grammar ends with a bare optional proof term:

syntax expected_smt_result "trace" ("[" ident "]")? "{" traceSpec "}" (term)? : command

set_option X in <term> is valid term syntax, so after } the parser consumes the following command as the trace's proof term, commits past in, and fails on sat ("expected spec" is the furthest-progress expectation, from the scoped spec : term rule). The consequences:

• the preceding trace command fails to parse as a whole, so it never runs and its result message is lost (the info above is from the second trace only);
• error recovery re-runs the inner command as a bare command, without the option applied;
• a term-shaped line after a trace parses successfully and is silently elaborated as the trace's proof term (a stray trivial after a trace reports an unrelated "Type mismatch").

This PR requires the proof term to start on the same line as the closing brace:

withPosition("}" (lineEq term)?)

The module above then parses and both traces run, with the option applied to the second. VeilTest/Regression/TraceTermTail.lean covers the trace-then-scoped-trace pattern, and the full lake build (Veil + VeilTest) passes.

Two notes:

1. Proof terms that started on the line after the } no longer parse. I found no in-tree usage; nothing in VeilTest or Examples passes a proof term to a trace.
2. A bare lineEq without an enclosing withPosition is a silent no-op, since there is no saved position to compare against. pickExpression in Veil/Frontend/DSL/Action/Syntax.lean uses kw_pick (lineEq term)? without one, so its guard may be vacuous as well; happy to look at that separately if useful.