perf: inline Source access in parser hot paths

Replace the Parser delegation layer (self._current, self.inc(),
self.mark(), self.extract(), self.end()) with direct access to the
Source object (src = self._src; src._current, src.inc(), etc.) in
all hot methods. This eliminates ~3M Python function calls per 500
parses — one extra frame per property access or method delegation.

The delegation wrappers are retained for use by less performance-
sensitive code paths.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: dimbleby