docs/entropy.md's boot-seed recipe calls sha256_of, which the packaged object does not export

`docs/entropy.md` tells a `RAND=drbg` integrator to build the boot seed by layering several sources and compressing them:

> Mix all of the following into the boot seed (concatenate and hash with `sha256_of`, or XOR into the reseed input).

That recipe does not link. `sha256_of` is not in `PUBLIC` (Makefile:227), and `objcopy $(foreach s,$(PUBLIC),-G $(s))` (Makefile:262) makes every other symbol local in the packaged object.

## Reproduction

```
$ make lib-check RAND=drbg
lib-check: 5 exported symbols, all public API

$ nm bin/chapulin.o | grep -E 'sha256_of|drbg_seed'
0000000000007e10 T _ch_drbg_seed
0000000000000b2c t _sha256_of <- local
```

Writing the page's own recipe and linking it against the object:

```
$ cc -I. -o seedtest seedtest.c bin/chapulin.o
Undefined symbols for architecture arm64:
"_sha256_of", referenced from:
_boot in seedtest.o
ld: symbol(s) not found for architecture arm64
```

The same file links cleanly with the `sha256_of` line removed, so that symbol is the only unresolvable one. (`ch_assert_fail` is supplied by the image as usual; the test defines it.)

## Why it matters

Compression is not optional on this path. `ch_drbg_seed` takes exactly 32 bytes (drbg.h:26), and the same page forbids seeding from one source alone. So an integrator following the documentation has to compress several sources into 32 bytes, and the call the page names for it is unavailable.

## Fix, two options

1. Export it under the drbg arm only: `PUBLIC_RAND := ch_drbg_seed ch_sha256`, renamed for the `ch_` prefix every public call carries. `lib-check` already reads a different export list per arm, so this costs the default arm nothing and keeps the four-export claim for builds that do not use the DRBG. It does gate an export on need rather than on dependency, one step past `x509_ca.h`'s precedent, so it deserves a decisions.md entry.
2. Rewrite the recipe to use only exported calls.

Either way, add a link-only check to the `lib-check RAND=drbg` leg that compiles and links the documented recipe, so it cannot rot again. Nothing in the tree currently compiles what the documentation tells an integrator to write, which is why this went unnoticed.

Found while evaluating https://github.com/c4milo/chapulin/issues/25.

Start with docs/entropy.md, the PUBLIC export lists and lib-check rules in Makefile, and the 32-byte API in drbg.h. Compare the documented boot-seed recipe with the RAND=drbg packaged object, then use the existing link-check pattern to verify that the documented recipe resolves and the documentation matches the supported API.

Written by the indexing model from the issue text.