Compiled regex patterns are rendered incorrectly in debug_repr output

[amol-]

Goal

Render compiled regex patterns correctly in debug_repr output: escape the
pattern for HTML and restore the quoting that repr() provides.

Done when: debug_repr(re.compile(...)) renders the pattern as readable text
for any pattern content, and a test covers a pattern containing markup
characters and a bytes pattern.

Reproduction (reproduces on Python 3.9 through 3.14):

import re
from backlash.repr import debug_repr
print(debug_repr(re.compile('<img src=x>')))
print(debug_repr(re.compile(b'a&b')))
print(debug_repr(re.compile('\n')))

Current output:

re.compile(<span class="string regex">r'<img src=x>'</span>)
re.compile(<span class="string regex">r'b'a&b''</span>)
re.compile(<span class="string regex">r'
'</span>)

Three separate defects, all from the same expression:

• The pattern is not passed through escape(), so <img src=x> is emitted as
  raw markup and the browser consumes it as a tag instead of displaying it. The
  user sees the pattern silently disappear from the console output.
• A bytes pattern renders as the nonsense r'b'a&b'', because str() is
  applied to the bytes object and then wrapped in another set of quotes.
• A pattern containing a newline emits a literal newline inside the quotes,
  breaking the single-line rendering the <span> is styled for.

Cause: backlash/repr.py builds the markup with
're.compile(<span class="string regex">r\'%s\'</span>)' % obj.pattern, using
the raw pattern with neither escape() nor repr().

Suggested minimal fix: render escape(repr(obj.pattern)) instead, which
restores quoting and escapes the markup in one step, and matches how the
sibling methods in the same class already handle strings.

Why

debug_repr is what the interactive console prints, so a regex in scope is
displayed wrongly or not at all. A pattern is exactly the kind of value someone
inspects while debugging a matching problem, and silently dropping part of it
(or showing r'b'a&b'') actively misleads.

The other repr methods in the same class already escape their output, so this is
an inconsistency within one class rather than a design choice.

Reachability note: debug_repr is only called from backlash/console.py, so
this affects the interactive console rather than the plain traceback page.

Verified as pre-existing rather than a regression: the pre-modernization code
had separate Python 2 and Python 3 branches here, and only the Python 2 branch
applied repr(); the Python 3 branch never escaped either.

References

• backlash/repr.py (DebugReprGenerator.regex_repr; py3_text_repr and
  py3_binary_repr in the same class show the escaping pattern used elsewhere)
• backlash/console.py (_ConsoleFrame/stream write path that calls debug_repr)
• backlash/utils.py (escape)
• tests/test_repr.py may be a natural home for a regression test