fix: strengthen Map/Set key hashing against collision DoS

[mvanhorn]

Summary

Strengthens the hashing of Map/Set keys against collision-based denial of service. String keys with embedded NUL bytes and float64 keys with adversarial bit patterns previously funneled into a single bucket, making insertion quadratic. Keys now hash with an avalanche mix so bucket distribution stays even under attacker-chosen inputs.

Why this matters

Issue #205 reports that Map/Set insertion can be driven quadratic with crafted keys. Two families were exploitable: strings sharing a prefix up to a NUL byte, and float64 values whose 32-bit halves are equal and share low bits (easy to generate via a DataView). Both collapsed to the same low hash bits, so once the table grew to 4096 buckets they all landed in one bucket and each insert walked the whole chain.

The string path is hardened, and the float64 key hash now runs the 64-bit pattern through a multiply/xorshift avalanche folded to 32 bits before the bucket mask, so equal-word / shared-low-bit families no longer collide. NaN normalization and +0/-0 unification are preserved.

Testing

make, make test (Result: 0/64 errors), and build/run-test262 -c tests.conf -f tests/test_builtin.js all pass. tests/test_builtin.js gains regression coverage for NUL-prefixed string keys, the equal-word float collision family at 4096 entries (both Map and Set), NaN, and +0/-0.

Fixes #205

[saghul]

time to drop the comment now?

[saghul]

Is this some well-known formula? maybe add a comment with a reference pl