det translator: a NOT ON SIZE ERROR after a nested unterminated arithmetic statement binds to the outer one #4676

Problem

A conditional arithmetic statement nested in another's ON SIZE ERROR phrase, without its own END- terminator, takes the following NOT ON SIZE ERROR phrase in COBOL (the phrase belongs to the innermost unterminated statement). GnuCOBOL does that; the det translator gives the phrase to the outer statement, so the port runs different statements.

Repro (measured 2026-10-07, GnuCOBOL 3.1.2 -std=ibm; A = 1, B = 3, C = 300, Z0 = 0, R = 12.34, R2 = 1):

           COMPUTE R = A / Z0
             ON SIZE ERROR COMPUTE R2 = A / B * C
             NOT ON SIZE ERROR COMPUTE R2 = A / B * 7
           END-COMPUTE
           DISPLAY 'B1 ' R ' ' R2
           ...more statements, no period

Oracle: B1 01234 00233 and nothing after it (the NOT phrase belongs to the inner COMPUTE; END-COMPUTE closes the innermost COMPUTE, so the rest of the sentence sits inside the outer ON SIZE ERROR). Det port: B1 01234 09900, and it runs the statements after it.

With each nested statement terminated (ON SIZE ERROR COMPUTE ... END-COMPUTE NOT ON SIZE ERROR COMPUTE ... END-COMPUTE END-COMPUTE) both sides agree (pinned by #4655's test_det_size_error.py, B1).

Proposal

In det/stmt.py's scope rules, attach a conditional phrase (ON SIZE ERROR / NOT ON SIZE ERROR, AT END, INVALID KEY, ON OVERFLOW ...) to the innermost unterminated statement that can take it, and pair an END- terminator with the innermost open statement of its verb. Probably rare in real estates (code style terminates nested statements); a test from this repro either way.

Part of #4270.

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