fix(implementer): a stray closing fence is not file contents

[jeremymanning]

What happened

Canary PROJ-9999 on main at 4e4161c was kicked back at step 28. Two causes, both in changes made earlier in this series.

1. Reply recovery (#1573). Four implementer replies in a row (gpt-oss-120b) were correct YAML followed by one closing ``` with no opening fence, or a final ---. Strict parsing fails on either ("found character '`' that cannot start any token"; "expected a single document"). The fallback from #1573 then recovered the artifact but kept the stray line as the last line of the file, so compilable Python was refused:

[implementer] refusing to write 'code/run_totient_analysis.py': SyntaxError at line 317: invalid syntax

Line 317 was the fence. Before #1573 the fallback did not match this reply shape at all, so this path is a regression introduced there.

2. Pattern evidence (#1574). The collector listed all twelve files of a family but quoted only the first and last. The verifier rejected: "the provided evidence only shows the contents of two CSVs … cannot verify that each of them sums to its respective N".

Changes

• yaml_extract.strip_trailing_markers() drops blank, fence and separator lines from the end of a reply, flush-left only. parse_yaml_lenient applies it, so these replies parse as YAML with no fallback; _regex_parse_implementer applies it too. An indented fence inside contents is kept.
• _pattern_evidence quotes every file of a matched family when the family fits a 30 KB budget; a larger family quotes the first and last and states how many are listed but not quoted.

Tests

• New fixture: the canary's real reply. It parses, and its Python compiles.
• Six trailing forms (```, ```yaml, ---, ..., with blank lines, combined) written and executed through write_artifacts.
• Fallback plus trailing fence; indented fence kept as content.
• Small family quoted in full; 40-file family quotes the ends and reports 38 not quoted.

Contract, unit and integration suites: 8543 passed, 21 skipped. ruff==0.17.0 clean.

Not verified: a canary run with this change.

🤖 Generated with Claude Code

https://claude.ai/code/session_01V7QkYvK7jaE1QVY1ZMfAt2