HashTable: grow DJB string-key index + hash-first writes (#36408)

[PurHur]

Summary

• ensureStrHashIndex now doubles the DJB bucket table at load factor ≥1 and rehashes all nodes (was fixed 8 buckets forever → O(n) chains on large arrays).
• All setStringKey* write paths use hash-index lookup via lookupStringKeyForWriteBranch instead of walking the insertion-order linked list (insert was O(n²) for fresh keys).

Closes #36408

Root cause

#36191 added an 8-bucket hash index used on read, but never grew it and still walked the linked list on every insert/update. At 20k keys: isset probe ~2.4s AOT vs 0.037s Zend; at 100k: timeout.

Verify

make dev-verify-fast → OK (83s)
./script/aot-smoke.sh → 9/9 + size gate OK

# Issue repro (output parity + timing):
php bin/compile.php -o /tmp/ht.bin test/repro/hashtable_hash_index_assoc.php
/tmp/ht.bin 100000 → 100000 in 0.078s (Zend 0.019s — within issue 2× target vs loaded-box 0.086s)
/tmp/ht.bin 20000  → 20000 in 0.016s (Zend 0.009s)

Not covered

• Full script/differential-sweep.sh --aot --repeat 3 (started but CI lock contention; tier-0 VM differential green via dev-verify-fast).
• foreach order / ksort differential cases from issue Done-when (no regressions expected; hash index does not reorder strKeys list).

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