Implement Hash::Util::lock_keys restricted hashes

[fglock]

Summary

Implement restricted hashes for Hash::Util::lock_keys (and the corresponding
query/unlock operations). PerlOnJava currently registers lock_keys but
returns without changing the hash, on both the JVM and interpreter backends.

Expected Perl behaviour

use Hash::Util qw(lock_keys);

my %hash = (foo => undef);
lock_keys(%hash);

eval { my $value = $hash{bar} };
print $@;

On standard Perl, the access dies with:

Attempt to access disallowed key 'bar' in a restricted hash

Internals::SvREADONLY(%hash) is true after lock_keys, and permitted keys
can be inspected with Hash::Util::legal_keys.

Actual PerlOnJava behaviour

The same program succeeds with no exception on both backends. The hash is not
marked readonly/restricted and its key set is not enforced.

The current Java implementation labels lock_keys and unlock_keys as
placeholder implementations. This also explains the existing feature-matrix
entry that marks restricted hashes unsupported.

CPAN evidence

Types::ReadOnly 0.003 failed in CPAN tester run 20260827-171032-2227.
Its Locked[...] constraints use Hash::Util::lock_keys plus
Internals::SvREADONLY to recognize restricted hash references. The affected
tests were t/03locked.t, t/10combinations.t, t/20moo.t, t/30moose.t,
and t/50typeparams.t: 13 of 60 subtests failed across 5 of 8 test programs.

Ecosystem impact

MetaCPAN reports 79 unique Hash::Util reverse-dependant distributions, 12
updated in the preceding 12 months. That total covers all Hash::Util APIs,
not only restricted hashes. Inspection of five representative dependants found
active lock_keys use in:

• DBIx-DataModel runtime schema metadata, to prevent typo-created keys.
• Net-Amazon-S3 shipped API/client parameter-validation helpers.
• CPAN-Testers-ParseReport option-parsing utility.

The same sample found only non-restricted APIs in Sereal-Encoder
(num_buckets test) and Function-Parameters (fieldhash test).

Suggested acceptance coverage

• Lock an existing hash and reject reads, writes, and deletes for disallowed
  keys using Perl-compatible diagnostics.
• Support explicitly declared allowed-but-absent keys.
• Make legal_keys report the declared key set.
• Verify unlock_keys restores ordinary hash behaviour.
• Ensure Internals::SvREADONLY reports the restricted-hash state expected by
  Type::Tiny/Types::ReadOnly.
• Exercise equivalent behavior on the JVM and interpreter backends.