[earthtojake]

Follow-up to #708. That PR caught f-strings that nest their own quote, a pattern Python 3.11 rejects but that ast.parse(feature_version=(3, 11)) on 3.12+ accepts. Review found three more PEP 701 forms that slip past the floor check the same way, plus a few nits. This closes them in the same token walk, with no CI changes.

What the check now flags (only when requires-python is below 3.12)

Each case below was checked against a real CPython 3.11 compile():

• A field reuses its f-string's quote (Catch same-quote nested f-strings in the 3.11 floor check #708's rule): f"{d["k"]}"
• A backslash in a field's expression: f"{'\n'.join(a)}", which 3.11 rejects with "expression part cannot include a backslash". A backslash in the literal text or the format spec is still allowed.
• A comment in a field: f"""{a # why\n}""". 3.11 rejects this even inside a triple-quoted f-string.
• A line break in a field of a single-quoted f-string: f"{a +\n b}". A triple-quoted f-string may still break lines in its fields.
• Fields nested more than two deep: f"{a:{b:{c}}}", which 3.11 rejects as "nested too deeply".

Nits from the #708 review

• One line with several offending strings is reported once.
• The module docstring no longer says feature_version answers "would the floor accept this file?" for f-strings.
• _quote no longer strips the t/T prefixes of t-strings, which it never tracks.

Other 3.12+ syntax is already rejected by feature_version=(3, 11). That covers PEP 695 type parameters and type aliases, and 3.14's t-strings.

Testing

• A 54-case harness compared the real 3.11 verdict with (feature_version or this check) on 3.12 and 3.13, with 0 mismatches. A separate set of 22 cases, written independently, also gave 0 mismatches on both.
• For each new rule, adding its offending line to a cadgen module made the floor test fail and name the file and line. The test passes on today's main.
• scripts/test/test-global.sh: 224 tests, OK (1 skipped). On 3.11 the pinning test skips, because the interpreter rejects all of this syntax itself.

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