fix(linting): single-export-per-file false positives on @fusionElement pattern

[odinr]

Why

Fixes #5185.

The single-export-per-file rule flagged the standard @fusionElement custom-element registration pattern:

export const tag = 'fwc-button';

@fusionElement(tag)
export default class _ extends ButtonElement {}

with '_' is the 2nd export in this file, even though the file conceptually only exports one thing (the tag const purely parameterizes the default class). Verified identically across ~30 packages in fusion-web-components.

Also, the index.ts barrel exemption only applied when options.match was left unconfigured — any repo (like fusion-web-components) supplying its own match/ignorePatterns config lost that exemption and got flagged on every barrel file.

What

• packages/linting/rules/src/single-export-per-file/index.ts:
  • Barrel files (index.ts, index.tsx, index.mts, index.cts) are now always exempt, ANDed with any options.match override rather than being replaced by it.
  • Added competingExports(): when a file has an export default class ... {}, companion top-level const/let exports are dropped from the competing-exports count before the one-symbol check runs.
• Added regression tests, including the exact case from the issue that export class Foo {} + export default Foo; (re-export, not a genuine 2nd export) continues to report 0 violations, and that a genuinely competing export alongside the pattern still gets flagged.

Validation

• npx vitest run packages/linting/rules/src/__tests__/single-export-per-file.test.ts — 19/19 passing.
• pnpm exec biome lint + pnpm exec fusion-lint lint on both changed files — clean.
• Added a changeset (patch for @equinor/fusion-framework-lint-rules).

[changeset-bot]

🦋 Changeset detected

Latest commit: 62f8cec

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

This PR includes changesets to release 1 package

Name	Type
@equinor/fusion-framework-lint-rules	Patch

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[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	65.59%	3106 / 4735
🔵	Statements	65.01%	3732 / 5740
🔵	Functions	50.79%	1080 / 2126
🔵	Branches	55.17%	1706 / 3092

File Coverage

File	Stmts	Branches	Functions	Lines	Uncovered Lines
Changed Files
packages/linting/rules/src/single-export-per-file/index.ts	96.22%	83.33%	100%	100%	129, 169

Generated in workflow #15035 for commit 62f8cec by the Vitest Coverage Report Action