add pass_filenames_via_stdin for large changesets

pre-commit currently passes selected filenames to hooks via argv.
For large changesets (or --all-files), argv length limits are hit and
filenames are partitioned, causing multiple hook invocations.

This means there is currently no built-in way to pass filenames to an
underlying hook in one shot without chunking / re-running. The only practical
workaround is to set pass_filenames: false and run custom git operations in
hook code to reconstruct the file set, which is expensive and duplicates
pre-commit's own file-selection logic.

This change adds a hook option:

    pass_filenames_via_stdin: true

When enabled, pre-commit sends filenames as NUL-delimited bytes on stdin and
runs the hook in a single invocation (no argv chunking).

Why NUL-delimited stdin:
- safe for filenames containing spaces/newlines
- matches established -0 conventions in unix tooling

Usage for hook authors:
- shell:

    while IFS= read -r -d '' filename; do
        ...
    done

- python:

    data = sys.stdin.buffer.read()
    filenames = [os.fsdecode(p) for p in data.split(b'\0') if p]

Behavior notes:
- default remains argv-based passing
- pass_filenames: false still disables filename passing entirely

Implementation includes schema/runtime wiring, shared NUL encode/decode
helpers, and tests covering defaulting and runtime behavior.

tests/lang_base_test.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import os.path
+import shlex
 import sys
 from unittest import mock
 
@@ -166,6 +167,42 @@ def test_basic_run_hook(tmp_path):
     assert out == b'hi hello file file file\n'
 
 
+def test_basic_run_hook_passes_filenames_via_stdin(tmp_path):
+    ret, out = lang_base.basic_run_hook(
+        Prefix(tmp_path),
+        (
+            f'{shlex.quote(sys.executable)} -c '
+            '\'import sys; '
+            'print(repr(sys.argv[1:])); '
+            'print(repr(sys.stdin.buffer.read()))\''
+        ),
+        (),
+        ['file1', 'file2'],
+        is_local=False,
+        require_serial=False,
+        color=False,
+        pass_filenames_via_stdin=True,
+    )
+    assert ret == 0
+    out = out.replace(b'\r\n', b'\n')
+    assert out == b"[]\nb'file1\\x00file2\\x00'\n"
+
+
+def test_to_nul_delimited_filenames():
+    ret = lang_base.to_nul_delimited_filenames(('file1', 'file2'))
+    assert ret == b'file1\x00file2\x00'
+
+
+def test_to_nul_delimited_filenames_empty():
+    ret = lang_base.to_nul_delimited_filenames(())
+    assert ret == b''
+
+
+def test_from_nul_delimited_filenames():
+    ret = lang_base.from_nul_delimited_filenames(b'file1\x00file2\x00')
+    assert ret == ['file1', 'file2']
+
+
 def test_hook_cmd():
     assert lang_base.hook_cmd('echo hi', ()) == ('echo', 'hi')