Closes the third sub-issue from PR #639's matrix-expansion follow-up
cluster (#641 cp1252 file I/O encoding) and flips the Windows
matrix entries from advisory to strict.

This is the "now works on Windows" release.

Set PYTHONUTF8=1 in the CI test job environment so Python's
text-mode open() defaults to UTF-8 regardless of locale (PEP 540).
Covers all CI-side encoding failures with a single config change
rather than auditing ~30 individual call sites.  Closes ~9 failing
tests across test_codegen.py, test_codegen_monomorphize.py,
test_codegen_closures.py, test_html.py.

Also added explicit encoding='utf-8' to vera/parser.py's grammar
load — the load-bearing site that runs on every parse, so users
on Windows without PYTHONUTF8=1 in their shell still get correct
grammar loading (defense-in-depth).

Filed #645 for the broader audit (every text-mode open() /
read_text() / write_text() call site for explicit encoding) as
the durable fix.

Drop continue-on-error advisory mode

PR #639 added Windows entries with continue-on-error: ${{ matrix.os
== 'windows-latest' }} so failures didn't block merges while #640,
sub-issues now closed (PR for #642 sanitises Windows paths in
test fixtures; PR for #640 reads sys.stdin cross-platform; this
PR closes #641).  The continue-on-error line is removed in this
release; the matrix is now fully strict across all 9 entries
(ubuntu / macos / windows × 3.11 / 3.12 / 3.13).

Version bump 0.0.142 → 0.0.143

This is the Windows-compatibility milestone — the project went
from "untested on Windows" to "Windows is a strict gate" in three
sequential PRs (#643 / #644 / this).  Marking with a minor version
bump and "now works on Windows" framing in CHANGELOG.

Co-Authored-By: Claude <noreply@anthropic.invalid>

The post-#637 Windows CI rollout (PRs #639/#643/#644/#646) hit
three test-fixture footguns — `tempfile.NamedTemporaryFile`'s
delete-while-open behaviour, Windows paths embedded in Vera
string literals tripping the `\U` escape, and cp1252 default
file I/O encoding.  Each had a portable workaround.

Added a "Test Fixture Conventions" section to TESTING.md
(immediately before "Adding Tests") with the three rules + their
canonical wrong/right code snippets.  Each rule cites the test
that surfaced the bug + the PR that fixed it.

Added a brief "Cross-platform pitfalls" section to CLAUDE.md's
project-orientation footer that points at the TESTING.md section.
The CLAUDE.md entry is a one-bullet pointer per rule, not a
full repeat — keeping the substantive content in TESTING.md as
the single source of truth.

These notes will save the next contributor (or agent) from
re-discovering the same footguns.

Co-Authored-By: Claude <noreply@anthropic.invalid>