#305 Introduce generic polymorphic type-mapping attributes

[sanelli]

Summary

• Add sealed generic helpers MappaTypeMappingAttribute<TTarget, TSource> and MappaTypeMappingDefaultAttribute<TDefault> (MapSourceType) that derive from the existing attributes; non-generic APIs remain.
• Teach AttributeDataExtensions to parse the generic attribute forms and reconstruct the same non-generic models.
• Cover with Mappa.Tests / Generator.Tests (including duplicated polymorphism integration tests), update samples and docs, bump to 10.2.0-alpha.58.

Test plan

• .\Scripts\RunTestsAndReportCoverage.ps1 — line/branch/method greater than or equal to pre-change baseline
• Confirm [MappaTypeMapping<TTarget, TSource>] generates the same switch mapping as the typeof form
• Confirm [MappaTypeMappingDefault<TDefault>] matches MapSourceType with an explicit target type
• Samples PolymorphismMapper / PolymorphismMapperWithMapDefaultWithExplicitType compile and pass sample tests

[github-actions]

Code coverage

Compared against the latest published coverage history from the code coverage gist.

Metric	Baseline	PR	Δ	Trend
Line	98.3%	98.3%	0	=
Branch	90.4%	90.4%	0	=
Method	100%	100%	0	=

No coverage metric dropped by more than 1 percentage point(s) versus baseline.

Benchmarks

Winners (chart subset)

Benchmark	Best time	Best memory
Array To List	Mapperly (-0.5%)	Mapperly, Mappa
List To Array	Mappa	Mapperly, Mappa
Fast List To Array	Mapperly (-10%)	Mapperly, Mappa
Dictionary	Mapperly (-21.2%)	Mapperly (-32%)
Objects mapping	Mapperly (-4%)	Mapperly (-5.4%)
IQueryable Projection	Mapster (-1.3%)	Mapster (-2.8%)
String To Enum	Mappa	n/a
Enum To String	Mapperly (-31%)	n/a
Reference Reusing	Mappa	Automapper (-17.3%)

AutoMapper is the BenchmarkDotNet ratio baseline. Absolute values are mean time in nanoseconds and allocated bytes. Ratio columns are competitor / Mappa as a percentage (n/a when either side is missing or Mappa is zero).

Benchmark	Automapper Mean (ns)	Automapper Allocated (B)	Mapster Mean (ns)	Mapster Allocated (B)	Mapperly Mean (ns)	Mapperly Allocated (B)	Mappa Mean (ns)	Mappa Allocated (B)	Automapper/Mappa Time	Automapper/Mappa Alloc	Mapperly/Mappa Time	Mapperly/Mappa Alloc	Mapster/Mappa Time	Mapster/Mappa Alloc
ArrayToListBenchmark	377360	592056.32	322160	648058.88	24770	128051.2	24900	128051.2	1515.5%	462.4%	99.5%	100%	1293.8%	506.1%
DictionaryBenchmark	386270	615014.4	362850	742277.12	37450	151019.52	47500	222218.24	813.2%	276.8%	78.8%	68%	763.9%	334%
EnumToStringBenchmark	66.236	24	20.749	24	0.501	0	0.726	0	9123.4%	n/a	69%	n/a	2858%	n/a
FastListToArrayBenchmark	372000	592025.6	314680	648028.16	24780	128020.48	27530	128020.48	1351.3%	462.4%	90%	100%	1143%	506.2%
IQueryableProjectionBenchmark	640700	68034.56	628900	67266.56	633400	69201.92	637400	69201.92	100.5%	98.3%	99.4%	100%	98.7%	97.2%
ListToArrayBenchmark	372380	592025.6	318850	648028.16	24930	128020.48	24900	128020.48	1495.5%	462.4%	100.1%	100%	1280.5%	506.2%
NestedDtoBenchmark	417660	742809.6	389370	789555.2	39900	149401.6	41570	157900.8	1004.7%	470.4%	96%	94.6%	936.7%	500%
ReferenceReusingSharedDagBenchmark	333.7	344	200.4	400	304.9	800	110.6	416	301.7%	82.7%	275.7%	192.3%	181.2%	96.2%
StringToEnumBenchmark	57.633	0	91.088	216	3.547	0	3.544	0	1626.2%	n/a	100.1%	n/a	2570.2%	n/a