rng module: portable PRNG plus entropy hook

New module, motivated by three separate real problems rather than a hypothetical:

1. FLAG S-01: numx_sketch_rsvd fails 2 ESP32-S3 tests because rand() seed portability differs across libc implementations (see validation/results/sketch/sketch.md). Not a bug in the algorithm, a portability problem in relying on the platform's rand().
2. Kyber keygen (the pqc module, #62) needs a CSPRNG. A zero-dependency, no-OS library can't assume /dev/urandom or any platform entropy source exists.
3. Every module that currently needs randomness (sketch's rsvd, this) rolls its own seeding ad hoc.

Proposed shape: a deterministic core PRNG (PCG or xorshift, small state, no allocation) for reproducible/testable use, plus a caller-supplied entropy callback (numx_rng_seed_from_entropy(fn) or similar) for the cryptographic use case in pqc, where the caller is responsible for supplying real entropy from whatever the platform provides.

Follow-up once this lands: #64 (migrate rsvd to it, close S-01).

Start with the portability failure described in validation/results/sketch/sketch.md and review the proposed numx_rng_seed_from_entropy(fn) API alongside the pqc module in #62. Define the deterministic PRNG core and caller-supplied entropy hook without allocation or OS dependencies; done means the new rng module supports reproducible use and entropy injection for cryptographic callers.