extphprs / ext-php-rs / 24421185838

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perf: pre-compute mangled property names in get_properties

The get_properties handler (called by var_dump, print_r, json_encode,
serialization) was calling format!() per property per invocation to
build PHP-convention mangled names. This allocated on every call.

PHP itself computes mangled names once at class registration time
(zend_mangle_property_name in zend_compile.c) and reuses them forever.

This commit brings the same approach:
- Field properties: the proc macro emits the mangled name as a
  compile-time string literal in PropertyDescriptor. Zero runtime cost.
- Method properties: mangled names are computed once on first access
  and cached in ClassMetadata via OnceCell. One allocation per class
  for the entire process lifetime.
- get_properties handler reads pre-computed names instead of format!().

Benchmark results (callgrind, 100k iterations):
  property_dump: -21.4% instructions, -20.0% estimated cycles
  property_read: +1.4% (struct size overhead, acceptable)
  property_write: +1.6% (struct size overhead, acceptable)

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