fix: collapse module into single file and remove lazy import

[redtux]

Summary

The two-file architecture (spdx_markdown.py + _impl.py) with a __new__-based
lazy wrapper is solving a circular-import problem that does not exist, and
introduces a real isinstance() bug.

Current behavior

1. SPDXMarkdown is a thin wrapper that resolves to _SPDXMarkdownImpl at
   instantiation time via __new__. The real class lives in _impl.py.
2. _SPDXMarkdownImpl does not subclass SPDXMarkdown, so
   isinstance(formatter, SPDXMarkdown) returns False — a latent bug for any
   code relying on type checks.

Root cause

The split was motivated by a perceived circular import: the claim was that the
entry-point scanner would deadlock if spdx_markdown.py imported
commitizen.changelog_formats.markdown at module level. In reality:

• Markdown is already in sys.modules when commitizen scans
  commitizen.changelog_format entry points — commitizen needs it internally
  for the built-in "markdown" format.
• Nothing in commitizen.changelog_formats imports back into this package.
  There is no cycle.

Proposed fix

Collapse the two files into one. Remove the __new__ redirection, all
TYPE_CHECKING stubs, and the _resolve() machinery. SPDXMarkdown becomes a
direct subclass of Markdown.

See Plan 10 — Collapse _impl.py for the
full implementation plan.

Additional scope

• Verify py.typed (PEP 561 marker) lands in the built wheel.
• Run ruff check src/ after the swap — the staggered-import pattern was the
  sole cause of existing E402 violations.

Reference

• Plan: docs/plans/10-collapse-impl.md