warn on filtered user traces

[knoal]

Summary

• warn when --trace-filter compiler-generated removes a user-defined trace
• explain that this filter can change evaluation behavior, not just emitted trace messages
• add regression coverage for the warning behavior

Fixes #1326.

Context

The current behavior reported in #1326 still reproduces on main: a user trace wrapped around a helper call can change the observable result under --trace-filter compiler-generated.

A maintainer comment on the issue notes that this is arguably the intended behavior of the filter, but also that it deserves at least a warning from the compiler.

This PR takes that narrower path.

What changed

• add a new type warning for user-defined traces compiled under Tracing::CompilerGenerated(..)
• emit the warning at trace-inference time on the trace @"...": ... expression itself
• document in the warning help text that compiler-generated trace filtering may alter evaluation semantics, not only which traces are displayed
• add tests proving:
  • the warning appears under Tracing::CompilerGenerated(..)
  • it does not appear under Tracing::All(..) or Tracing::UserDefined(..)

Validation

• cargo test -p aiken-lang compiler_generated_filter_warns_for_user_trace -- --nocapture
• cargo test -p aiken-lang no_filtered_user_trace_warning_without_compiler_generated_filter -- --nocapture
• manual repro via:
  • cargo run -q -p aiken -- check /tmp/aiken-1326-repro --seed 1 --max-success 1 --trace-level verbose --trace-filter compiler-generated

The manual repro now still shows the existing pass/fail behavior, but also surfaces a warning directly on the filtered trace @"user trace": ... expression.