test: drive the direct executor end to end, and fix four rendering defects

[thedancingdeveloper]

tests/test_executor_e2e.py drives the direct executor the way run drives
it — claim, plan, implement, apply, check, review, commit — against a real
temporary git repository, a real git apply, real check commands that
are shell scripts, and a scripted model transport. It asserts on the resulting
git state and queue row, never on internals. Same pattern as
tests/test_agent_loop_e2e.py, pointed at the other executor.

It found four bugs, all in the seam D10 opened on 2026-08-05: the implementer
now returns edit blocks and edits.to_diff renders them into a unified diff.
The rendering is new; everything downstream of it is not. git apply compares
bytes, so a diff that is wrong about the file is refused and no rung of the
tolerance ladder can help — which is the good news and the bad news. Every
one of the four is an edit the model got right, lost between matching the
text and applying the patch.

The four

1. A file whose last line has no newline produced a corrupt patch.
   difflib signals a missing final newline by emitting a line that does not
   end in one. Concatenated straight into the diff, -beta and +gamma
   arrive as -beta+gamma and git reports corrupt patch at line 6. Now
   rendered with \ No newline at end of file, and the file keeps its shape
   byte for byte.

2. A CRLF file could never be edited at all. read_text translates on the
   way in, so the SEARCH block matches happily; the diff was then rendered in
   LF and refused with patch does not apply. Nothing the model could write
   would have changed that — the item failed identically on every attempt
   until its ladder ran out. The file's line ending is now read off disk and
   the diff is rendered in it.

3. An empty file that exists was rendered as a creation. fromfile was
   chosen on whether the content was empty, so an empty __init__.py got
   --- /dev/null and git refused with already exists in working directory
   — while plan_edits deliberately permits an empty SEARCH against exactly
   that file. It now asks whether the file exists.

4. The diff was computed against the working tree, which holds the previous
   item's branch. select_repo_context already refuses to read the tree for
   this reason and reads through git show base:path instead; to_diff read
   the tree. So the second item to touch any file named text it had been
   shown, that is present on the base its branch will be cut from, and was
   told the SEARCH text does not occur in the file. The model was right and
   the harness blamed it — and the better the previous item did, the more
   certain the next one was to fail. The tree is now moved to the base before
   the implementer is asked. No branch is created there, so an item that
   produces no usable diff still leaves none behind.

No gate is weakened

Nothing here makes anything more tolerant. Three of the four fixes make the
rendered diff more faithful to the file on disk, which is what lets the
strictest rung — plain git apply — accept it; the fuzzy rung stays off and
--unidiff-zero is not relied on. The fourth removes a disagreement between
two parts of the harness about which tree an item is being written against.

What the tests also pin, and found sound

Checks run before the reviewer is paid (asserted by the reviewer never being
called); a rejected review keeps its checkpoint commit; nothing lands on the
default branch; a refused edit costs an attempt and leaves the tree clean with
no branch; ambiguous SEARCH text is refused rather than applied to the first
match; edit blocks are applied in order so a later one may depend on an
earlier one's result; a unified diff is still accepted as a fallback; edits
change nothing is reported as no change rather than as a broken patch; #152's
ceiling blocks an item without consuming an attempt and without the
implementer ever seeing an unseen target; and #155's declared fix clears its
gate, reaches the commit, and is stated to the reviewer — while a fix that
does not clear its gate still refuses the item.

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