perf(@angular/build): use native sass-embedded compiler daemon

[alan-agius4]

Replaces the Piscina-based worker thread pool executing pure-JS Dart Sass
(sass compiled via dart2js) with a native Dart Sass asynchronous compiler
daemon (sass-embedded).

The build system previously relied on multi-threaded worker pools with synchronous
compilation to bypass performance overheads inherent to pure-JS asynchronous Dart
Sass. However, this approach incurred significant V8 heap and isolate duplication
across worker threads, as well as thread contention and blocking due to
Atomics.wait() synchronization loops during custom URL and module rebasing.

With sass-embedded, Sass compilation is delegated to a long-lived native Dart
AOT binary communicating asynchronously via Protocol Buffers over stdio pipes.
This eliminates duplicate V8 isolates, replaces synchronous Atomics.wait()
with event-driven asynchronous message handling, and dramatically reduces memory
pressure on the Node.js process.

The compiler infrastructure is structured around a shared SassServiceImplementation
interface with two isolated implementations dynamically loaded on demand:

• SassAsyncCompilerImplementation: Manages the native sass-embedded daemon lifecycle
  and asynchronous URL rebasing importers (AsyncModuleUrlRebasingImporter).
• SassWorkerImplementation: Retained as an isolated worker pool fallback.

Environment Variable & Fallbacks

• NG_BUILD_SASS_WORKER=1 (or true): Forces the build system to fall back to
  the pure-JS SassWorkerImplementation using worker threads.
• In restricted environments where native platform binaries cannot be spawned
  (such as WebContainers detected via process.versions.webcontainer), the build
  system automatically falls back to SassWorkerImplementation without requiring
  manual configuration.

Benchmarks: Sass Service in Isolation

10 Small Files (Angular Components)

Metric	Worker Pool (main)	Native Async (this change)	Delta	Speedup / Improvement
Cold Start	369.04 ms	229.89 ms	-139.15 ms	1.61x faster (-37.7%)
Warm Median	22.46 ms	5.27 ms	-17.19 ms	4.26x faster (-76.5%)
Warm Mean	25.82 ms	7.25 ms	-18.57 ms	3.56x faster (-71.9%)
Warm P95	37.30 ms	14.55 ms	-22.75 ms	2.56x faster (-61.0%)
Warm Throughput	445.2 files/s	1,899.3 files/s	+1,454.1 files/s	4.27x throughput (+326.6%)
Incremental Median	6.54 ms	2.08 ms	-4.46 ms	3.14x faster (-68.2%)
Incremental Throughput	153.0 files/s	480.5 files/s	+327.5 files/s	3.14x throughput (+214.0%)
Peak Memory RSS	281.16 MB	78.73 MB	-202.43 MB	-72.0% memory saved

10 Large Files (Design System Tokens & Themes)

Metric	Worker Pool (main)	Native Async (this change)	Delta	Speedup / Improvement
Cold Start	823.95 ms	386.25 ms	-437.70 ms	2.13x faster (-53.1%)
Warm Median	231.85 ms	175.64 ms	-56.21 ms	1.32x faster (-24.2%)
Warm Mean	241.50 ms	172.22 ms	-69.28 ms	1.40x faster (-28.7%)
Warm P95	293.09 ms	215.48 ms	-77.61 ms	1.36x faster (-26.5%)
Warm Throughput	43.1 files/s	56.9 files/s	+13.8 files/s	1.32x throughput (+32.0%)
Incremental Median	72.63 ms	35.34 ms	-37.29 ms	2.06x faster (-51.3%)
Incremental Throughput	13.8 files/s	28.3 files/s	+14.5 files/s	2.05x throughput (+105.4%)
Peak Memory RSS	986.47 MB	112.34 MB	-874.13 MB	-88.6% memory saved

Benchmarks: Real Angular CLI Application Scenarios (ng build)

Small App (5 small files)

Phase / Metric	Main (NG_BUILD_SASS_WORKER=1)	This Change (Native Async)	Delta	Improvement
Cold Wall Time	3,201.3 ms	2,950.2 ms	-251.1 ms	-7.8%
Cold CLI Time	2,456.0 ms	2,302.0 ms	-154.0 ms	-6.3%
Cold Peak RSS	1,227.3 MB	1,058.9 MB	-168.4 MB	-13.7%
Warm (Avg) Wall Time	2,762.0 ms	2,475.3 ms	-286.7 ms	-10.4%
Warm (Avg) CLI Time	2,071.3 ms	1,820.3 ms	-251.0 ms	-12.1%
Warm (Avg) Peak RSS	1,102.3 MB	894.8 MB	-207.5 MB	-18.8%

Small App (5 large files)

Phase / Metric	Main (NG_BUILD_SASS_WORKER=1)	This Change (Native Async)	Delta	Improvement
Cold Wall Time	3,605.9 ms	3,175.8 ms	-430.1 ms	-11.9%
Cold CLI Time	2,874.0 ms	2,482.0 ms	-392.0 ms	-13.6%
Cold Peak RSS	1,420.2 MB	1,072.3 MB	-347.9 MB	-24.5%
Warm (Avg) Wall Time	3,123.7 ms	2,682.5 ms	-441.2 ms	-14.1%
Warm (Avg) CLI Time	2,435.7 ms	2,028.7 ms	-407.0 ms	-16.7%
Warm (Avg) Peak RSS	1,278.2 MB	911.3 MB	-366.9 MB	-28.7%

Large App (500 files: 250 large + 250 small)

Phase / Metric	Main (NG_BUILD_SASS_WORKER=1)	This Change (Native Async)	Delta	Improvement
Cold Wall Time	15,259.7 ms	15,348.6 ms	+88.9 ms	+0.6% (Parity)
Cold CLI Time	14,425.0 ms	14,373.0 ms	-52.0 ms	-0.4% (Parity)
Cold Peak RSS	2,305.1 MB	1,582.7 MB	-722.4 MB	-31.3% memory saved
Warm Run 1 Wall Time	14,673.1 ms	14,277.1 ms	-396.0 ms	-2.7%
Warm Run 1 CLI Time	13,812.0 ms	13,375.0 ms	-437.0 ms	-3.2%
Warm Run 1 Peak RSS	2,180.5 MB	1,485.6 MB	-694.9 MB	-31.9% memory saved
Warm Run 2 Wall Time	14,771.2 ms	14,502.7 ms	-268.5 ms	-1.8%
Warm Run 2 CLI Time	13,943.0 ms	13,527.0 ms	-416.0 ms	-3.0%
Warm Run 2 Peak RSS	2,220.8 MB	1,493.3 MB	-727.5 MB	-32.8% memory saved
Warm (Avg) Peak RSS	2,200.7 MB	1,489.5 MB	-711.2 MB	-32.3% memory saved

Closes #29319

[gemini-code-assist]

Code Review

This pull request introduces support for the persistent Dart Sass embedded compiler (sass-embedded) via a new SassAsyncCompilerImplementation. It refactors the existing worker-based Sass compilation into SassWorkerImplementation and introduces a conditional check (useSassWorker) to choose between the native asynchronous compiler and the worker-based fallback. Feedback on the changes highlights potential issues with error handling during asynchronous initialization and shutdown, specifically recommending try-finally blocks to ensure failed initialization promises are cleared so subsequent attempts can retry, and a try-catch block in close() to prevent initialization failures from disrupting the shutdown sequence. Additionally, it is suggested to validate importers upfront in SassAsyncCompilerImplementation to match the fail-fast behavior of the worker implementation.

[alan-agius4]

This PR was merged into the repository. The changes were merged into the following branches:

• main: ecbcd87