[Fixed][Windows] Layers cache written with \r\r\n row endings breaks cached diffs

[mike-shevchenko]

Problem

On Windows, the second diff of any board fails:

File "kicad-diff.py", line 793, in load_layer_names
    layer_names, name_to_id = load_cached_layers(layers_file)
File "kicad-diff.py", line 704, in load_cached_layers
    ilnum = int(r[0])
IndexError: list index out of range

The first diff of a given board always succeeds, because it is a cache miss and the layers
are read from the PCB. Every later diff that hits the cache fails, which in practice means
any persistent --cache_dir is unusable on Windows.

Cause

csv.writer always terminates rows with \r\n. save_layers_to_cache() opened the stream
in text mode without newline='', so on Windows the \n was translated a second time and
each row landed as \r\r\n:

$ od -c layers.csv | head -2
0000000   L a y e r   I D , L a y e r   n a m e , U s e r   n a m e  \r  \r  \n

Reading that back yields a blank row between each real one, and int(r[0]) raises
IndexError. This is the case the csv docs call out - newline='' is required on both the
read and the write side.

Fix

• newline='' on both open() calls, so rows are written and read as \r\n.
• Skip blank rows on read. Strictly this is not needed once the writer is fixed, but without
  it every cache already written by an earlier version keeps crashing until the user finds
  and deletes it. Two lines seemed better than a migration note.

No behaviour change on Linux or macOS, where \n is not translated and the bug never
appeared.

Verification

Same two boards, same cache directory, KiCad 10.0.6 / Python 3.11 on Windows 10:

	before	after
first diff (cold cache)	ok	ok
second diff (warm cache)	IndexError	ok
cache written by old version	IndexError	ok
layers.csv row ending	\r\r\n	\r\n

The last row is the backward-compatibility case: I deliberately rewrote a good cache back to
\r\r\n and confirmed the patched script reads it while the unpatched one still raises.