perf(compiler): bypass dynamic dispatch for known-array appends - #97

For known arrays, append expressions such as $items[] = makeValue() still use offsetSet(null, tmp) when the RHS needs materialization or the assignment result is used. That dispatches through runtime type/key checks before reaching Array::append.

Call append(tmp) directly for Type::ARRAY, retaining the existing RHS temporary and expression result. Dynamic receivers, simple statement fast paths, and explicit reference assignments retain their existing lowering.

Adds a code-generation regression test, an executable semantics test covering references, assignment results, copy-on-write and RHS mutation, and a standalone append benchmark with raw measurements and reproduction instructions.

Performance

On Linux ARM64 in Docker, PHP 8.5.10 ZTS, PHPX 6a68f38, GCC -O2 without LTO, against baseline 692841a6: 100 million appends took a median 980.88 ms before and 563.60 ms after (42.54% less elapsed time). All nine alternating measured pairs were faster after one discarded warm-up pair; checksums matched. This is an append-heavy microbenchmark on a shared development machine, not an application-wide speedup or a claim about native PHP performance.

Validation

• HotPathCodegenTest: 5 tests, 25 assertions pass; new codegen test fails against the original trait.
• Three targeted PHPTs pass: array-append-expression, array-statement-write-optimized, and array-value-write-dereferences-source.
• Baseline and candidate semantics outputs match PHP 8.5.10; native PHP benchmark checksum matches both compiled binaries.
• git diff --check passes. Full-suite and cross-platform validation were not run.