Preserve nested conditional values in comprehension elements

[EiNSTeiN-]

I am opening this PR to preserve nested conditional values in both arms of a comprehension element. The code below currently loses the entire comprehension and decompiles as return rows on Python 3.14:

def select(rows):
    return [{
        "excluded": ({"count": row["count"],
                      "reasons": loads(row["reasons"]) if row["reasons"] else {}}
                     if row["count"] is not None else {}),
        "archived": str(row["archived"]) if row["archived"] else None,
    } for row in rows]

The outer then arm constructs a dictionary containing another conditional expression. The pure-value scan handles nested conditionals in the else arm but can stop prematurely in the then arm. It also omits dedicated None-test jumps from its recognized conditional-jump family.

The change makes genexpr_ternary_merge recursively prove and skip a nested pure-value region in either arm, retaining the enclosing arm's actual final merge. This preserves the dictionary construction, decoding calls and their failures on the selected arm. A false outer condition still skips the inner decoding call entirely.

The same reconstruction handles nested conditional call arguments and None tests in either arm:

[build(1 if row.inner else 2) if row.outer else 3 for row in rows]
[1 if row.outer else build(2 if row.inner else 3) for row in rows]
[build(1 if row.inner is None else 2) if row.outer
 else build(3 if row.inner is not None else 4) for row in rows]

Conditional expressions begin in Python 2.5. This repair restores these examples on the Python 3.12+ inlined-comprehension path; older comprehension reconstruction still has gaps. docs/LIMITATIONS.md distinguishes the repaired nested-arm case from those remaining older-version limitations.