fix(postcss): preserve source escapes in changed fields

[Anber]

Motivation

When a fixer rewrites only part of a selector or declaration value, the field-wide source snapshot no longer matches. The stringifier therefore treats the entire field as newly generated and escapes source-derived backslashes again.

Closes #1508.

Summary

• expose the original field value to the stringifier
• tokenize original and changed fields into CSS escapes and surrounding text
• preserve retained escape tokens while still escaping newly introduced ones
• conservatively treat duplicate escapes as fixer-owned when their count changes
• bound the comparison size and fall back to the existing safe behavior
• cover declaration values, mixed selector escapes, and ambiguous duplicates

Validation

• 5 Jest suites, 100 tests, 13 snapshots
• focused stringify suite on Node.js 24
• TypeScript typecheck
• ESLint
• Prettier

[changeset-bot]

🦋 Changeset detected

Latest commit: 57c31cb

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 9 packages

Name	Type
@linaria/postcss-linaria	Patch
@linaria/atomic	Patch
@linaria/babel-plugin-interop	Patch
@linaria/core	Patch
@linaria/react	Patch
@linaria/server	Patch
@linaria/stylelint	Patch
@linaria/stylelint-config-standard-linaria	Patch
linaria	Patch

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