AOT: fix concat ephemeral temp lifetime for dead echo/call operands (#23798)

[PurHur]

Summary

• Dead php-cfg Concat temporaries (echo/call args) were lowered in-place into scope slots that can alias the concat left operand, corrupting the local on a second concat from the same variable.
• Route operands in deadOperands through assignEphemeralConcatOperand: fresh __string__* in a dedicated entry alloca (KIND_VARIABLE) instead of in-place CONCAT or bare KIND_VALUE rvalues.
• Extends AOT: emit concat for unallocated call-arg Temporaries (#23779) #23792 (call-arg emit) to cover single-arg calls and consecutive echo $s.'1'; echo $s.'2';.

Verification

# Issue reproducers (30× repro2 loop — was ~45% heap corruption)
php bin/compile.php -o /tmp/repro1 build/micro/argloc/repro1.php && /tmp/repro1   # → s1
php bin/compile.php -o /tmp/repro2 build/micro/argloc/repro2.php && /tmp/repro2   # → s1 / s2

# AOT fixtures (Docker)
./script/phpunit.sh test/aot/AotTest.php --filter concat_inline
# OK (3 tests, 15 assertions): concat_inline_call_args, concat_inline_call_arg_single, concat_inline_echo_same_var

Not run: script/differential-sweep.sh --aot (VM unaffected per issue), bootstrap/self-host gates (no compile-spine change).

Closes #23798

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