test: make three fixtures portable so they test the code, not the host platform

[Hotragn]

Summary

Five Windows test failures are caused by how the fixtures build their input, not by the code under test. The source is already correct in all three cases. This makes the fixtures portable so they test the code rather than the host platform.

Windows goes from 13 failed / 550 passed to 8 failed / 555 passed. No behaviour change on macOS or Linux — every edit changes only how a fixture serializes its own input.

Why

1. test_tagging.py — 3 failures. Five page fixtures write content with write_text, which translates \n to os.linesep, then assert byte-exact round-tripping with read_bytes(). On Windows the intended \r\n lands on disk as \r\r\n, which is not a valid frontmatter fence — so the page looks like it has no frontmatter, a second block gets prepended, and the original is absorbed into the body:

'---\ntopics:\n- auth\n- sessions\n---\n---\r\r\ntitle: Auth Flow\r\r\n...'
 ^^^^^^^^^^^ new block                 ^^^^^^^^^^ original, now body text

Worth stating plainly: rewrite_page_topics is not at fault. It reads bytes, detects the line ending, and preserves it — I checked before assuming a source bug. newline="" simply writes what the test meant to write.

2. test_transcript_discovery.py — 1 failure. The Claude fixture hand-builds JSONL with an f-string:

f'{{"sessionId":"claude-1","cwd":"{repo}"}}\n'

On Windows repo interpolates as C:\Users\..., so the JSON contains invalid escapes (\U, \A), the line fails to parse, and discover() returns 0 candidates.

Its Codex sibling in the same file already uses json.dumps and passes on Windows for exactly that reason. This just adopts the pattern already established two functions up; json was already imported.

3. test_build_workflow.py — 1 failure. The expected value mixes separators and ignores that the prompt is JSON-encoded:

assert f'"manual_root": "{repo}/almanac/manual"' in adapter.requests[0].prompt

On Windows repo renders with backslashes, then /almanac/manual is appended with a forward slash, while the prompt contains the JSON-escaped native path. Building the expected value with json.dumps and a pathlib join compares like with like.

Verification

# Windows 11, Python 3.13 (uv-managed), uv 0.12.0
uv run pytest tests/test_tagging.py tests/test_transcript_discovery.py \
              tests/test_build_workflow.py -q
# 21 passed

uv run pytest -q          # 8 failed, 555 passed, 1 skipped
# baseline on main:         13 failed, 550 passed, 1 skipped

uv run ruff check .       # All checks passed!
git diff --check          # clean
uv build --out-dir dist   # ok

Also confirmed green on Linux semantics by inspection: newline="" is a no-op where os.linesep is already \n, and json.dumps of a POSIX path is byte-identical to the previous f-string output.

Docs and wiki

• Not applicable — test fixtures only.

Notes for reviewers

• This is the last thing standing between the suite and a meaningful Windows CI job. After this plus fix(tests): make isolated_home actually isolate, and apply it to every test #64 and fix(paths): reject every path anchor in repo-relative guards, not just absolute ones #65, the only remaining Windows failures are the 5 macOS-only launchd tests. That is the point at which adding windows-latest to ci.yml produces a signal instead of noise — which is the substance of the proposal in Proposal: one platform seam plus a CI matrix, so cross-platform PRs become reviewable #67. This PR is useful on its own and does not depend on that discussion going anywhere.
• I fixed 5 sites in test_tagging.py, not the 3 that fail. Lines 47 and 86 pair the same translating write with a read_bytes assertion and pass today only because their assertions happen not to be suffix-exact. Leaving them would be leaving the same bug armed.
• No src/ change, deliberately. I checked each source path before touching the test, specifically so this would not turn into a false bug report against working code. frontmatter_rewrite.py is the model for byte-exact read-modify-write in this codebase.
• Sent separately from fix(tests): make isolated_home actually isolate, and apply it to every test #64/fix(paths): reject every path anchor in repo-relative guards, not just absolute ones #65/fix(wiki): skip uppercase markdown suffixes so a .MD file cannot break every query #66 because it shares no files with any of them and can merge in any order.

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