[TrimmableTypeMap] Optimize build times

[simonrozsival]

Part of #10788

Track build-time optimization opportunities for the trimmable type map generation pipeline (GenerateTrimmableTypeMap task).

Current performance (PR #10924 benchmarks, MacBook M1)

Phase	Avg	P50	P95
Scan (8870 types)	235ms	224ms	319ms
Emit (typemap assemblies)	86ms	75ms	181ms
JCW (315 Java files)	766ms	409ms	3167ms
Total cold	1088ms	702ms
Total incremental (scan only)	221ms	215ms

Optimization opportunities

• Parallelize JCW source generation — each Java file generation is independent; running them in parallel could significantly reduce the JCW phase which dominates cold build time (suggested by @elinor-fung in [TrimmableTypeMap] Add GenerateTrimmableTypeMap MSBuild task and targets #10924 (comment))
• Parallelize assembly scanning — scanning assemblies for Java peers could be parallelized across assemblies (suggested by @elinor-fung in [TrimmableTypeMap] Add GenerateTrimmableTypeMap MSBuild task and targets #10924 (comment))
• Use MSBuild partial builds — replace the in-task timestamp checking with MSBuild native Inputs/Outputs 1-1 mapping so MSBuild only passes changed assemblies to the task, eliminating the stamp file (suggested by @jonathanpeppers in [TrimmableTypeMap] Add GenerateTrimmableTypeMap MSBuild task and targets #10924 (comment))
• Cache scan results across incremental builds — currently all assemblies are rescanned every build for cross-assembly type resolution; caching the scan index or adding a two-list Scan overload could skip unchanged assemblies
• Skip unchanged JCW files — avoid writing Java files whose content hasn't changed (attempted but read+compare cost negated savings; may be worth revisiting with content hashing)

[simonrozsival]

PR #10959 adds per-assembly acw-map.{AssemblyName}.txt files as a side-output of GenerateTrimmableTypeMap. When optimizing incremental builds (e.g., per-assembly Inputs/Outputs or the two-list Scan overload), the acw-map files should be included in the incrementality story:

• If an assembly is unchanged and its scan is skipped, the per-assembly acw-map file should also be preserved (not regenerated)
• AcwMapWriter.WriteToFile already does content comparison to avoid unnecessary timestamp updates, but skipping the write entirely (when the scan is skipped) would be even better
• The _MergeAcwMaps target uses Inputs/Outputs on the per-assembly files, so it will automatically skip if no acw-map file was touched

Adding this as a consideration item for the incremental build optimization work.