code_size: membrowse link_command crashes the run on non-UTF-8 ninja output #4087

Originating PR: #4086

code_size's membrowse engine crashes the whole run instead of failing per elf when the ninja -t commands output is not UTF-8 (Windows, ninja < 1.11 or a non-UTF-8 ninja -t wincodepage).

Evidence (verified at PR head e538050, severity low):

• tools/membrowse_cli.py:28 (unchanged by code_size: run report and diff on Windows #4086, present on master c391fe9): subprocess.run([ninja, '-C', build_dir, '-t', 'commands', '-s', rel], capture_output=True, text=True). Under PYTHONUTF8 (required on Windows) text=True decodes strictly as UTF-8.
• CMake v4.1.2 writes build.ninja in the ANSI code page when ninja < 1.11 or ninja -t wincodepage is not UTF-8 (cmGlobalNinjaGenerator.cxx:814-822, 829-872, 1086-1087). A non-ASCII checkout path (the -T linker script lives under it) then yields non-UTF-8 output.
• On Windows Popen decodes in a reader thread: the UnicodeDecodeError is printed by threading.excepthook and r.stdout is None, so r.stdout.strip() (line 33) raises AttributeError (UnicodeDecodeError on POSIX). Neither is RuntimeError, so it passes report()'s per-elf except RuntimeError in tools/code_size.py, escapes pool.map, and generate_sizes only catches FileNotFoundError. The whole run ends in a traceback instead of a per-elf error naming the remedy.
• code_size: run report and diff on Windows #4086 documents the setup as unsupported (code-size SKILL.md: a non-ASCII checkout, CMake or toolchain path needs ninja >= 1.11 whose ninja -t wincodepage reports UTF-8) and gives build.ninja reads an explicit RuntimeError with that remedy. This path in link_command is not covered.

Remaining work: in link_command, turn a decode failure (UnicodeDecodeError/AttributeError, or read bytes and decode explicitly) into a RuntimeError with the same ninja >= 1.11 remedy. No encoding fallback (maintainer decision: Windows requires PYTHONUTF8, no encoding code). Add a unit test.

Why deferred: the line predates #4086 and that PR's scope is the Windows enablement it adds; the setup is documented unsupported, so the impact is a traceback instead of the documented message.