fix(cksum): reject absurdly large SHAKE --length instead of aborting

[koopatroopa787]

Fixes

Fixes #12869

Problem

`cksum -a shake128/shake256 --length` accepted any `usize` value in bits with no upper bound. For example:

```
$ cksum -l=10011111117721172727 --algorithm shake128 a
memory allocation of 1251388889715146591 bytes failed
Aborted (core dumped)
```

The unsanitized length flows straight into the digest buffer allocation in `Digest::result()` (`vec![0; self.output_bits().div_ceil(8)]`), which aborts the whole process on an allocation failure rather than returning a clean error.

Fix

Cap `--length` for SHAKE128/SHAKE256 at 8192 bits (1 KiB of output), comfortably above any realistic use case, and report a GNU-style error (`invalid length: '...'` + `maximum digest length for 'SHAKE128' is 8192 bits`) when exceeded — mirroring the existing bound already enforced for BLAKE2b.

Test plan

• Added test_shake_length_too_large_does_not_panic covering both shake128 and shake256.
• cargo test --test tests shake passes (29 tests).
• Manually verified the original repro from the issue now exits 1 with a clean error instead of aborting.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/symlink (passes in this run but fails in the 'main' branch)

[koopatroopa787]

Done — reworked the fix as suggested. Digest::result() now returns io::Result<DigestOutput>, and all implementations (including the impl_digest_shake! macro) allocate the output buffer via a new try_reserve_zeroed helper that uses Vec::try_reserve_exact and maps an allocation failure to an io::Error with ErrorKind::OutOfMemory, instead of imposing an arbitrary upper bound. The one production call site (digest_reader in checksum/mod.rs) now propagates this with ?.

Dropped the fixed-cap test and replaced it with one that just verifies an absurdly large --length fails cleanly instead of aborting the process, without asserting a specific cap value.

Pushed as c3bb6a7.

[oech3]

Thankyou!