fix: preserve trace SVG byte contract on Windows

[Anionex]

Problem

On Windows, Path.write_text() translates each SVG \n to \r\n, while trace reports len(svg). Multi-line output therefore reports fewer bytes than were written, and strict consumers reject the artifact. len(svg) is also a character count rather than a UTF-8 byte count.

Downstream context: Anionex/dsh-vision-toolkit#8.

Fix

• encode the finalized SVG once as UTF-8 and write those exact bytes with Path.write_bytes()
• report the actual byte count returned by the write operation, keeping the on-disk artifact and stdout contract identical
• cover multi-line LF preservation and non-ASCII byte counting without requiring optional tracing dependencies
• run the focused trace regression in both the Ubuntu core matrix and the Windows CI job

Verification

• python3 -m py_compile vision_proxy.py vision_client.py ground.py detect.py bin/glance bin/trace bin/crop
• required core tests: image rewrite, focus hint, Anthropic rewrite, proxy smoke, and vision client — passed
• python3 tests/test_trace.py — focused unit regression passed
• uv run --with pillow --with vtracer python tests/test_trace.py — full trace CLI path passed
• git diff --check
• GitHub CI: Python 3.11, Python 3.14, Windows, and extensions — passed

This is a non-UI CLI contract fix; there is no browser path. The byte-for-byte assertion and Windows CI execution cover the affected behavior.