[browser] WebAssembly SDK targets more incremental

[maraf]

Summary

Improves MSBuild incrementalism for WebAssembly browser build targets in Microsoft.NET.Sdk.WebAssembly.Browser.targets. On no-op rebuilds where inputs haven't changed, the expensive ConvertDllsToWebcil and GenerateWasmBootJson tasks are now skipped via MSBuild's Inputs/Outputs mechanism.

Changes

Boot JSON Generation (commit 1)

Split _GenerateBuildWasmBootJson into 3 targets:

• _ResolveBuildWasmBootJsonEndpoints (always runs) -- resolves endpoints and fingerprinted assets
• _WriteBuildWasmBootJsonFile (incremental) -- writes boot JSON file only when inputs change
• _GenerateBuildWasmBootJson (always runs) -- defines static web assets from the boot JSON output

Split GeneratePublishWasmBootJson into 2 targets:

• _ResolvePublishWasmBootJsonInputs (always runs) -- resolves publish endpoints
• GeneratePublishWasmBootJson (incremental) -- writes boot JSON only when inputs change

Webcil Conversion (commit 2)

Split _ResolveWasmOutputs into 3 targets:

• _ComputeWasmBuildCandidates (always runs) -- resolves build asset candidates via ComputeWasmBuildAssets
• _ConvertBuildDllsToWebcil (incremental) -- runs ConvertDllsToWebcil only when DLL inputs are newer than webcil outputs
• _ResolveWasmOutputs (always runs) -- reconstructs webcil items via MSBuild item transforms and calls DefineStaticWebAssets

Incrementalism Proof

Binlog analysis comparing baseline (no changes, no-op rebuild) vs PR branch (no-op rebuild with outputs up-to-date):

Target                                            Base       PR     Change
-------------------------------------------------------------------------
_GenerateBuildWasmBootJson                       144ms      4ms
_WriteBuildWasmBootJsonFile                        N/A     SKIP     NEW+SKIPPED
_ResolveBuildWasmBootJsonEndpoints                 N/A     38ms     NEW
_ResolveWasmOutputs                              175ms    103ms
_ConvertBuildDllsToWebcil                          N/A     SKIP     NEW+SKIPPED
_ComputeWasmBuildCandidates                        N/A     SKIP     NEW+SKIPPED

Baseline: 12 targets ran, 2 skipped, 1445ms total WASM target time
PR:       13 targets ran, 5 skipped, 1289ms total WASM target time
Reduction: ~10.8% in WASM target execution time

Key savings on incremental rebuilds:

• ConvertDllsToWebcil task (~35ms) -- skipped when webcil outputs are newer than DLL inputs
• GenerateWasmBootJson task (~95ms) -- skipped when boot JSON is newer than all assembly/JS inputs

Clean builds show negligible overhead (~1%, within noise).

Design Notes

Why split instead of just adding Inputs/Outputs?

MSBuild's Inputs/Outputs mechanism skips the entire target body when outputs are up-to-date. Since _ResolveWasmOutputs and _GenerateBuildWasmBootJson both contained file-writing tasks AND item-defining tasks (DefineStaticWebAssets), making them incremental would break downstream targets that depend on the items they produce. The split separates file I/O (incremental) from item definitions (always-run).

MSBuild Condition evaluation order

_ConvertBuildDllsToWebcil has Condition="'$(_WasmEnableWebcil)' == 'true'" but MSBuild evaluates Condition before DependsOnTargets. Since _WasmEnableWebcil is set by _ResolveWasmConfiguration (inside _ComputeWasmBuildCandidates), the parent _ResolveWasmOutputs explicitly lists _ComputeWasmBuildCandidates in its DependsOnTargets before _ConvertBuildDllsToWebcil.

Culture/non-culture DLL separation

Non-culture DLLs and culture-specific DLLs have different metadata (RelatedAsset only exists on culture items). Using %(RelatedAsset) in item transforms causes MSB4096 batching errors when applied to items without that metadata. The fix uses separate intermediate item names (_WasmWebcilConvertedNonCulture, _WasmWebcilConvertedCulture) to avoid cross-contamination.

Testing

• Clean build and publish succeed
• No-op rebuild produces correct output
• Binlog analysis confirms incremental targets skip when outputs are up-to-date

[Copilot]

Pull request overview

This PR improves MSBuild incrementalism for WebAssembly SDK build targets by splitting monolithic targets into smaller, more focused targets with proper Inputs/Outputs declarations. This allows MSBuild to skip expensive operations (webcil DLL conversion, boot JSON generation) on no-op rebuilds when inputs haven't changed.

Changes:

• The _ResolveWasmOutputs target is split into _ComputeWasmBuildCandidates (resolve/classify candidates), _ConvertBuildDllsToWebcil (incremental webcil conversion), and _ResolveWasmOutputs (reconstruct webcil metadata and define static web assets).
• The _GenerateBuildWasmBootJson target is split into _ResolveBuildWasmBootJsonEndpoints (resolve endpoints), _WriteBuildWasmBootJsonFile (incremental boot JSON generation), and _GenerateBuildWasmBootJson (define static web asset for boot config).
• The GeneratePublishWasmBootJson target is split into _ResolvePublishWasmBootJsonInputs (resolve inputs) and GeneratePublishWasmBootJson (incremental publish boot JSON generation).