add opacity calculation to Particle System 2D

[zxx43]

Re: # & #

Changelog

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Continuous Integration

This pull request:

• needs automatic test cases check.

  Manual trigger with @cocos-robot run test cases afterward.

• does not change any runtime related code or build configuration

  If any reviewer thinks the CI checks are needed, please uncheck this option, then close and reopen the issue.

---

Compatibility Check

This pull request:

• changes public API, and have ensured backward compatibility with deprecated features.
• affects platform compatibility, e.g. system version, browser version, platform sdk version, platform toolchain, language version, hardware compatibility etc.
• affects file structure of the build package or build configuration which requires user project upgrade.
• introduces breaking changes, please list all changes, affected features and the scope of violation.

[github-actions]

Code Size Check Report

Wechat (WASM)	Before	After	Diff
2D Empty (legacy pipeline)	1015436 bytes	1015436 bytes	✅ 0 bytes
2D All (legacy pipeline)	2683233 bytes	2683579 bytes	⚠️ +346 bytes
2D All (new pipeline)	2775492 bytes	2775838 bytes	⚠️ +346 bytes
(2D + 3D) All	10032934 bytes	10033280 bytes	⚠️ +346 bytes

Web (WASM + ASMJS)	Before	After	Diff
(2D + 3D) All	16869352 bytes	16869698 bytes	⚠️ +346 bytes

Interface Check Report

! WARNING this pull request has changed these public interfaces:

@@ -70482,8 +70482,14 @@
         export class _cocos_particle_2d_particle_simulator_2d__Particle {
             pos: math.Vec2;
             startPos: math.Vec2;
             color: math.Color;
+            curColor: {
+                r: number;
+                g: number;
+                b: number;
+                a: number;
+            };
             deltaColor: {
                 r: number;
                 g: number;
                 b: number;

[zxx43]

@cocos-robot run test cases

[github-actions]

@zxx43, Please check the result of run test cases:

• Test Platform: PR-Test
• Editor Version: v4.0.0
• Task Result: PASS
• Task URL: https://cctest.cocos.org/#/reportview/PR-TEST/c9c89405-c9cb-4c07-926e-e6b68efce40b/-1
• GitHub Action: https://github.com/cocos/cocos4/actions/runs/32805017803

Task Details

[github-actions]

@zxx43, Please check the result of run test cases:

• Test Platform: PR-Test
• Editor Version: v4.0.0
• Task Result: PASS
• Task URL: https://cctest.cocos.org/#/reportview/PR-TEST/c9b324ef-27d5-46bc-b528-6d751b827fa3/-1
• GitHub Action: https://github.com/cocos/cocos4/actions/runs/32805017803

Task Details

[LinYunMo]

Similar to spine fix, it adds the ability to cascade transparency to components.

[llsansun]

This extra color structure is needed for precision. Since Color stores RGBA as 8-bit values, using a float curColor prevents precision loss during per-frame color interpolation.