fix(export): retain RGBA alpha with presentation opacity

[rudycelekli]

What does this PR do?

Combine RGBA color alpha with an existing numeric presentation opacity exactly once. Preserve RGBA when CSS or other-element opacity could affect the unresolved cascade, keeping appearance without claiming strict-consumer compatibility in that fallback.

Updates the packaged helper, existing regression verifier/test, affected import/export reference and README together. No manifest version, IR field or diagram type change.

Visual proof

A direct half-alpha/half-opacity red shape exports as alpha64, matching its source. Actual CSS, inline and inherited opacity controls also retain source alpha64 through conservative RGBA preservation; before that fallback they incorrectly exported alpha128.

The proof uses owned neutral source or unchanged shipped assets and actual native producer/browser callers. No diagram, example or template variant is edited; no application integration beyond the named producer/browser is claimed.

Validation gates

• All 77 commands from .maintainer-policy.json passed on the exact signed head.
• python3 scripts/test-export-svg-standalone.py passed, including the new regression and existing controls; the regression fails against the original production implementation.
• Native producer/public CLI evidence passed.
• Documentation updated with behavior and limits.

Tested head: 9d02c195530360c716c564fdde2eefdda67f227f. Local validation uses macOS, Python 3.12.14, Playwright 1.62.0, Chromium 151.0.7922.34 and Pillow 12.1.1. Hosted matrix checks remain pending. Generated icon outputs passed the existing EOL-insensitive gate, then were restored to their index line endings.

Checklist

• No manifest version changes or new skin baseline entries.
• Generated/source artifacts remain consistent.
• Existing accessible SVG and unchanged shipped variants retain mandatory gates.
• Code of Conduct respected.

Related issues

Closes #288.

Codex assisted with investigation, implementation and verification on behalf of @rudycelekli. Companion fixes address separate reproduced failures but share helper/verifier/reference sections and may require integration. Existing #282/#284 parsing work, #249 font prose and #66 broad role prose are separate scopes.

Review regressions cover parent fill/stroke inherited independently from child paint, encoded inline declarations and escaped CSS property names. Seven actual source/export browser controls match on the current signed head. This converter does not claim to resolve the full CSS cascade or guarantee strict SVG 1.1 import for unresolved RGBA.

[greptile-apps]

Confidence Score: 3/5

[Medium risk] Changes SVG color export logic for opacity handling.

Fix the inherited-opacity and encoded-CSS cases before merging; both can change the exported appearance.

Findings

1.  Children become too transparent ▶
2.  Encoded CSS loses color alpha ▶

Summary

This PR combines RGBA alpha with numeric presentation opacity and preserves RGBA when it detects CSS or another element's opacity.

• Scopes paint changes to actual attributes and adds regression tests.
• Updates the README and export reference with the fallback limits.
• Two appearance problems remain: children inherit changed opacity, and encoded or escaped CSS declarations bypass the fallback.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A[RGBA paint attribute] --> B{Matching CSS or another opacity attribute found?}
  B -->|Yes| C[Keep RGBA]
  B -->|No| D{Own numeric opacity?}
  D -->|Yes| E[Multiply alpha and replace opacity]
  D -->|No| F[Add opacity from alpha]
  E --> G[Children inherit the changed opacity]
  F --> H[Missed CSS can override the added opacity]

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