Stdlib: round() mode: named arg skips omitted precision (#9564)

[PurHur]

Summary

• Fix round($num, mode: PHP_ROUND_HALF_*) when $precision is omitted: sparse named-arg maps bind mode at index 2, but count($calledArgs)===2 wrongly treated index 1 as precision.
• Use isset($frame->calledArgs[n]) per slot in VM round.php (same pattern as number_format.php) and NamedOptionalCallArgs::isOmittedOptional() in JitRound.php.
• Add maintainer repro + extend round_named_params.phpt with mode:-only case.

Closes #9564

Root cause

round.php used $argc >= 2 / $argc === 3 on a sparse named-arg array ([0 => num, 2 => mode]). count() is 2, so precision was read from the missing/wrong slot.

php-src reference

ext/standard/math.c — zif_round accepts optional precision + mode independently.

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php bin/vm.php test/repro/maintainer_gap_round_php_round_half.php'
# 3 / 2 / 4 / 3

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php bin/jit.php test/repro/maintainer_gap_round_php_round_half.php'
# 3 / 2 / 4 / 3

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php bin/vm.php -r "echo round(2.5, mode: PHP_ROUND_HALF_UP), \"\\n\";"'
# 3

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter round_named_params'
# OK (VM + JIT compliance)

Note: AOT round_precision_mode fixture was already red on master (unrelated LLVM round lowering); VM/JIT paths fixed here.

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