Improve pySpecs robustness: graceful failures, regex support, and reusable API

[joehendrix]

Summary

Improves the robustness of the pySpecs command so it reports failures
gracefully — including regex usage — instead of aborting, and extracts
the core logic into a reusable API with CLI flags for skipping
definitions, controlling verbosity, and selecting log events.

Details

• Graceful failure handling. When a Python import cannot be resolved,
  the translator now registers the imported names as opaque extern types
  and continues, rather than aborting. This means downstream references
  to those names still produce valid (if incomplete) output. Overload
  stubs with unexpected bodies produce a warning instead of failing.
• Regex support. from re import compile is recognized via the
  built-in prelude, so Python files that use regex no longer trigger
  unknown-identifier errors. A test confirms this import resolves
  without warnings.
• Reusable API. The translation logic that was inline in the CLI
  callback is extracted into a self-contained entry point in SimpleAPI.
  It validates inputs, parses skip-name strings, translates, writes
  output, and reports warnings — all returning structured errors instead
  of calling exitFailure. The CLI callback now delegates to this.
• CLI flags. --skip <name> omits named top-level definitions from
  output (overloaded variants are always kept). --quiet suppresses
  logging and warnings. --log <event> enables specific event types.
• Warning pipeline. Warnings are now collected during translation
  and returned to the caller as data, rather than being printed to
  stderr internally. A three-level verbosity setting (none / summary /
  detail) controls how they are reported.
• Composable assertion expressions. The expression tree for
  preconditions and postconditions is refactored from fused nodes
  (e.g., "length >= N" as one node) into orthogonal building blocks
  (length, integer literal, greater-or-equal, etc.). This makes
  serialization a direct structural recursion with no special cases.
• Readable DDM output. Assertions print as ensure(pred, msg).
  Comparisons use infix >=/<=. Class member blocks are indented
  on separate lines. These changes make the generated specs easier to
  read and diff.
• Event logging cleanup. Logging no longer uses a global mutable
  reference; the set of enabled events is threaded through the
  translation context. Import messages now report after completion
  (past tense) and include warning counts.

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[shigoel]

Nit: looks like fileStem can be used everywhere and stem isn't needed

[shigoel]

Out of scope for this PR, but noting a panic here that'd be a good candidate to convert to an exception.