MVP: write generics, compile, and deploy

[math3usmartins]

What's in it

Bundle (src/)

• XphpBundle (AbstractBundle) -- config tree (source / target / cache /
  hash_length / compile_on_warmup / register_runtime_autoloader / services)
  and the wiring that turns declared generics into DI services.
• XphpCompiler -- drives the transpiler's public console facade in-process;
  uniform CompilationFailedException on any failure.
• xphp:compile command and a mandatory, fail-loud cache warmer, so
  cache:warmup (run by every prod build) produces the generated PHP -- and a
  compile error aborts the build rather than shipping stale output.
• GeneratedClassLoader -- runtime SPL autoloader for the XPHP\Generated\*
  specializations (no composer dump-autoload needed in dev).
• GenericServiceRegistrar -- resolves each declared binding to the
  monomorphized class via the transpiler's own Registry::generatedFqn and
  registers it, honoring per-binding autowire / autoconfigure / public /
  shared / tags over inheritable _defaults.

Opt-in DI for generics

Declare instantiations under xphp.services.bindings; the bundle compiles + registers
them at container-build time (so they're reflectable for autowiring/autoconfigure):

xphp:
  services:
    bindings:
      - template: 'App\Generic\CachedFinder'
        args: ['App\Entity\User']

Because only .xphp can name a generic, the consumer of a Foo<Bar> is itself
.xphp (a service, command, or controller) and injects the specialization directly.

Known stopgap: monomorphization is usage-driven, so a config-only generic
(never new-ed in source) has no specialization to register. The bundle bridges
this by synthesizing a throwaway instantiation per binding at compile time
(transparent, cleaned up afterwards). The clean fix is an upstream "specialization
roots" API in xphp -- a follow-up.

Demo (demo/)

A console-only Symfony app consuming the bundle via a Composer path repository, showing
both patterns side by side:

• app:demo -- value-type generic: Box<Plastic> monomorphized and called from
  compiled code.
• app:find -- generic service via DI: an .xphp command autowires a
  CachedFinder<User> (whose own CacheInterface + source-service deps are
  autowired), demonstrating cache-aside.

Notes / follow-ups

• Library, so no composer.lock is committed (CI resolves latest compatible deps).
• The synthetic-roots stopgap -> upstream xphp "specialization roots" API.
• lazy / bind per-binding knobs deferred; the schema extends to them trivially.

How to try it locally

make test            # unit
make test/mutation   # mutation (100% MSI)
cd demo && composer install
bin/console app:find
bin/console app:demo