Add find_values() convenience method with optional native acceleration

[SereinCin]

Summary

Add a find_values() convenience method to the JSONPath base class that returns plain values (equivalent to [m.value for m in path.find(data)]). When the optional aero-jsonpath package is installed, find_values() delegates to a native Aero-compiled kernel for significant speedups on filter/descendant workloads.

Motivation

The standard find() returns DatumInContext objects, so extracting plain values requires a list comprehension every time. find_values() provides this as a one-liner and opens the door to optional native acceleration without changing the existing API.

Performance (benchmark: 2000-book document, full pipeline)

Expression	aero (q/s)	jsonpath-ng (q/s)	Speedup
$..author	261	44	5.98x
$..book[?(@.isbn)]	144	37	3.90x
$..book[0,1,2].title	191	51	3.77x
$.store.book[?(@.price < 15 & @.category == 'fiction')].title	278	110	2.54x
$.store.book[?(@.price < 10)].title	282	163	1.73x

Changes

• jsonpath_ng/jsonpath.py: add find_values() to JSONPath base class with native fallback
• jsonpath_ng/parser.py, jsonpath_ng/ext/parser.py: store source expression string on parsed path
• jsonpath_ng/_native.py: native accelerator bridge that imports aero_jsonpath when available
• tests/test_find_values.py: 6 tests covering filters, descendants, slices, string comparisons, AND conditions
• CHANGELOG.md: document the new method

Compatibility

• Fully backward compatible: existing find() API is unchanged
• find_values() falls back to [m.value for m in self.find(data)] when aero-jsonpath is not installed
• No new required dependencies

Test plan

All 357 existing tests pass + 6 new tests for find_values():

============================= 357 passed in 4.96s =============================