gh-156443: Keep PyLong loop carries as twodigits in shifts and division - #157060

…presses

Maintainer review fixes for PR gastownhall#217, iteration 1. The two gating defects are
both in coverage_contract.py — the artifact this PR designates the tiebreaker
— and both were reproduced with matched controls before being fixed.

F1 (major) — _strip_code substituted a space for a code span, and a space is
exactly the separator _CLOSING_KEYWORD_REF accepts between a keyword and its
reference. Removing inline code from "Fixes `x` gastownhall#42" therefore *created* the
keyword->reference adjacency GitHub does not honour: the stripper minted
mechanical `closing-keyword` coverage for a body that closes nothing. This is
the expensive direction — a body shaped like "fixes `--resume` #3849" makes
the tiebreaker instruct an agent to bury a live issue. Code is now replaced
by a NUL sentinel, which separates the two sides without ever joining them.

F2 (major) — the fence-close alternative is anchored on a MULTILINE `$` that
a trailing \r sits in front of, so a properly closed CRLF fence read as
unclosed and the `\Z` branch blanked every reference after it. CRLF is the
delivery form the GitHub API actually returns for web-authored bodies, and
"description, code block, Fixes #N at the bottom" is the standard template.
Line endings are now normalised before matching. Confirmed against the real
body review identified: python/cpython#157060 classified no-coverage-evidence
before this change and yields {156443} after. Folded in the same-character
fence-close rule while reworking the regex — a ``` block "closed" by a ~~~
line ended the blanked region early, the same phantom direction as F1.

F3 — mol-pr-triage's verdict branch asserted coverage_contract.py implements
"exactly these rules" while omitting the evidence floor its sibling states and
pins, so a triage agent following that prose could record a Tier-4 demotion
citing a span the shared contract rejects: a false Tier 4, the expensive
direction again, and the one finding all three review lanes filed
independently. The floor is now stated on both sides and mirrored against
_MIN_EVIDENCE_CHARS/_MIN_EVIDENCE_WORDS, and the baseRefName restriction names
$DEFAULT_BRANCH instead of leaving the fetched variable unused.

The suite passed on the shipped tree AND on every fix variant, so green tests
are not evidence here and each new test was verified by breaking the thing it
guards: reverting the sentinel, the CRLF normalisation, the fence
backreference, the floor sentence, the $DEFAULT_BRANCH binding, the repo=
forwarding, and the keyword separator each fail their own assertion. Neither
regression test passes on a stripper that simply deletes everything — the
positive arm is what rules that out.

Picked up while in these files: a gate-level test for
decide_competing_pr_gate(repo=), which 0 of its 13 call sites exercised; a pin
for the no-space `Fixes:gastownhall#42` form, recorded rather than endorsed so a future
relaxation is deliberate; and a fixture comment that paraphrased what it
presented as a verbatim quotation.

tests: 126 passed (was 120). All 6 pack formulas still parse. git diff --check
clean.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>