Optimize MessageDifferencer::TreatAsSet time complexity to O(N * K)

[copybara-service]

Optimize MessageDifferencer::TreatAsSet time complexity to O(N * K)

Optimizes MessageDifferencer::TreatAsSet() repeated field matching by using
hash map lookup (GetRepeatedElementHash) for unmatched elements.

Previously, set matching performed O(N^2 * K) element-pair comparisons. With
hash map lookup, set matching executes in average O(N * K) time complexity.
Updated documentation in message_differencer.h and added
TreatAsSet_LargeUnorderedSet test case to message_differencer_unittest.cc.

Mitigations:

• Uses absl::InlinedVector<int, 2> in hash map buckets to eliminate per-element heap allocations.
• Uses element count threshold (count > 4) to bypass hash map construction for small lists (N <= 4), avoiding hashing overhead on small element sets.
• Samples 32 bytes from the prefix and 32 bytes from the suffix (64 bytes total) for large strings, keeping string hashing within hardware CRC32/SIMD throughput while eliminating large payload throughput regressions.

Benchmark Replication Command:
benchy
--reference=<baseline_client>
--perflab
--cpu=haswell
--runs=6
--benchmark_filter=all
//experimental/users/dpages/message_differencer_benchmark:message_differencer_benchmark
--benchmark_enable_random_interleaving
--benchmark_repetitions=50

Benchy Results (Sponge: http://sponge2/d30b23f6-8606-4579-81bb-628600fd8d2e):
Generated by http://go/benchy on Perflab (Haswell, 6 runs, 50 interleaved repetitions).
Overall Geomean: 59.10 µs -> 18.64 µs (-68.45% CPU).

1. Repeated Scalar (Int32) - Permuted / Unordered Set Matching

N	Condition	Baseline CPU	Optimized CPU	Change	Speedup
10	Permuted	1.73 µs	1.22 µs	-29.74%	1.4x
50	Permuted	29.37 µs	5.09 µs	-82.67%	5.8x
100	Permuted	112.28 µs	9.84 µs	-91.23%	11.4x
500	Permuted	2.71 ms	50.23 µs	-98.15%	54.0x
1,000	Permuted	10.80 ms	100.6 µs	-99.07%	107.4x
5,000	Permuted	269.37 ms	504.1 µs	-99.81%	534.3x
10,000	Permuted	1,078.37 ms	1.04 ms	-99.90%	1,037.9x

2. Repeated String - Permuted / Unordered Set Matching

N	Condition	Baseline CPU	Optimized CPU	Change	Speedup
10	Permuted	2.26 µs	1.52 µs	-32.99%	1.5x
50	Permuted	38.06 µs	6.47 µs	-83.01%	5.9x
100	Permuted	146.64 µs	12.78 µs	-91.28%	11.5x
500	Permuted	3.53 ms	66.57 µs	-98.12%	53.1x
1,000	Permuted	14.37 ms	133.4 µs	-99.07%	107.7x
5,000	Permuted	352.79 ms	727.0 µs	-99.79%	485.3x

3. Repeated Scalar (Int32) - Mismatch / Unmatched Elements

N	Condition	Baseline CPU	Optimized CPU	Change	Speedup
10	Mismatched	1.16 µs	0.73 µs	-37.12%	1.6x
100	Mismatched	82.19 µs	5.34 µs	-93.51%	15.4x
1,000	Mismatched	8.09 ms	55.29 µs	-99.32%	146.3x
5,000	Mismatched	201.94 ms	276.6 µs	-99.86%	730.1x
10,000	Mismatched	808.08 ms	570.7 µs	-99.93%	1,415.9x

4. String Payload Size Scaling (N=50, Varying String Size)

(Bounded prefix+suffix hashing eliminates large payload throughput bottlenecks)

N	String Size	Baseline CPU	Optimized CPU	Change
50	16 B	19.43 µs	5.97 µs	-69.25%
50	64 B	20.23 µs	6.50 µs	-67.88%
50	256 B	21.37 µs	8.46 µs	-60.43%
50	1 KB	22.32 µs	9.88 µs	-55.73%
50	4 KB	31.45 µs	19.70 µs	-37.37%
50	16 KB	68.57 µs	53.64 µs	-21.77%
50	64 KB	181.7 µs	167.5 µs	-7.80%

5. Mostly Identical Elements (90% Identical in Order, 10% Permuted)

Benchmark	N	Baseline CPU	Optimized CPU	Change	Speedup
Int32_MostlyIdentical	10	660.9 ns	719.4 ns	+8.86%	~ parity
Int32_MostlyIdentical	100	5.64 µs	5.33 µs	-5.45%	1.1x
Int32_MostlyIdentical	1,000	151.35 µs	52.66 µs	-65.21%	2.9x
Int32_MostlyIdentical	5,000	2.91 ms	263.1 µs	-90.95%	11.1x
Int32_MostlyIdentical	10,000	11.20 ms	527.2 µs	-95.29%	21.3x
String_MostlyIdentical	10	816.5 ns	816.8 ns	+0.04%	~ parity
String_MostlyIdentical	100	7.53 µs	6.74 µs	-10.59%	1.1x
String_MostlyIdentical	1,000	203.80 µs	66.62 µs	-67.31%	3.1x
String_MostlyIdentical	5,000	3.91 ms	340.4 µs	-91.30%	11.5x

6. Duplicate & Identical Order Fast Paths

Benchmark	N	Condition	Baseline CPU	Optimized CPU	Change
Int32_IdenticalOrder	10	Prefix match (identical order)	662.1 ns	719.5 ns	+8.67%
Int32_IdenticalOrder	100	Prefix match (identical order)	4.57 µs	4.97 µs	+8.69%
Int32_IdenticalOrder	1,000	Prefix match (identical order)	43.59 µs	45.62 µs	+4.67%
Int32_IdenticalOrder	5,000	Prefix match (identical order)	217.1 µs	233.0 µs	+7.32%
Int32_IdenticalOrder	10,000	Prefix match (identical order)	434.4 µs	463.4 µs	+6.66%
String_IdenticalOrder	10	Prefix match (identical order)	819.0 ns	818.2 ns	-0.10%
String_IdenticalOrder	100	Prefix match (identical order)	6.07 µs	5.99 µs	-1.25%
String_IdenticalOrder	1,000	Prefix match (identical order)	59.50 µs	59.43 µs	~ parity
String_IdenticalOrder	5,000	Prefix match (identical order)	297.6 µs	298.2 µs	+0.22%
Int32_AllDuplicates	10	Duplicates (identical order)	661.2 ns	719.8 ns	+8.86%
Int32_AllDuplicates	100	Duplicates (identical order)	4.57 µs	4.97 µs	+8.73%
Int32_AllDuplicates	500	Duplicates (identical order)	21.91 µs	22.98 µs	+4.91%
Int32_AllDuplicates	1,000	Duplicates (identical order)	43.58 µs	45.61 µs	+4.66%
Int32_AllDuplicates	5,000	Duplicates (identical order)	217.1 µs	233.0 µs	+7.34%
String_AllDuplicates	10	Duplicates (identical order)	807.8 ns	824.2 ns	+2.04%
String_AllDuplicates	100	Duplicates (identical order)	6.52 µs	6.49 µs	~ parity
String_AllDuplicates	500	Duplicates (identical order)	34.27 µs	33.06 µs	-3.55%
String_AllDuplicates	1,000	Duplicates (identical order)	71.41 µs	69.02 µs	-3.35%
String_AllDuplicates	5,000	Duplicates (identical order)	366.5 µs	369.9 µs	~ parity

7. Small Element Count Fallback Threshold (N=2, Varying String Size)

(Tests N <= 4 threshold bypass to eliminate hashing overhead on small lists)

N	String Size	Baseline CPU	Optimized CPU	Change
2	16 B	384.3 ns	393.7 ns	+2.45%
2	64 B	384.7 ns	397.4 ns	+3.29%
2	256 B	407.1 ns	421.0 ns	+3.42%
2	1 KB	457.6 ns	485.7 ns	+6.13%
2	4 KB	639.1 ns	737.7 ns	+15.44%
2	16 KB	1.68 µs	1.70 µs	+1.10%
2	64 KB	5.19 µs	5.22 µs	+0.69%

8. Submessages & Baseline Controls

Benchmark	N	Condition	Baseline CPU	Optimized CPU	Change
Message_Permuted	2	Submessages (direct compare)	1.51 µs	1.51 µs	~ parity
Message_Permuted	5	Submessages (direct compare)	4.39 µs	4.38 µs	-0.10%
Message_Permuted	10	Submessages (direct compare)	12.43 µs	12.48 µs	+0.34%
Message_Permuted	50	Submessages (direct compare)	224.2 µs	227.1 µs	+1.30%
Message_Permuted	100	Submessages (direct compare)	856.8 µs	869.5 µs	+1.48%
Message_Permuted	500	Submessages (direct compare)	20.66 ms	20.98 ms	+1.55%
DefaultList_Identical	10	Non-set default list diff	397.3 ns	407.6 ns	+2.58%
DefaultList_Identical	100	Non-set default list diff	2.28 µs	2.29 µs	+0.31%
DefaultList_Identical	1,000	Non-set default list diff	20.95 µs	20.97 µs	~ parity
DefaultList_Identical	5,000	Non-set default list diff	104.0 µs	104.1 µs	~ parity
DefaultList_Identical	10,000	Non-set default list diff	207.9 µs	207.9 µs	~ parity