chore: fix readme and add copilot instructions

[GordonSmith]

Checklist:

• The commit message is properly formatted and free of typos.
  • The commit message title makes sense in a changelog, by itself.
  • The commit message includes a "fixes" reference if appropriate.
  • The commit is signed.
• The change has been fully tested:
  • I have viewed all related gallery items
  • I have viewed all related dermatology items
• There are no similar instances of the same problem that should be addressed
  • I have addressed them here
  • I have raised new issues to address them separately

Testing:

[Copilot]

Pull Request Overview

This PR updates the dataflow package documentation by correcting test file paths in the README and adding comprehensive Copilot instructions for package contributors.

• Updates all test reference links in README from old src/__tests__/*.ts paths to new tests/*.spec.ts paths
• Adds a new .github/copilot-instructions.md file documenting architecture, patterns, and development workflows

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
packages/dataflow/README.md	Updates 28 test file reference links to point to the correct tests/*.spec.ts location after test directory restructuring
packages/dataflow/.github/copilot-instructions.md	Adds comprehensive documentation covering architecture, coding patterns, build workflows, and TypeScript configuration

---

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[Copilot]

The claim that sort.ts uses arguments.length optimization is inaccurate. The actual implementation in sort.ts still uses isSource(s_or_cb) for runtime type checking, not arguments.length. This documentation should either reference a different file that actually uses this pattern, or acknowledge that this optimization is planned but not yet implemented.

Suggested change

	**Performance optimization (in progress):** Activities are being migrated from `isSource()` runtime checks to `arguments.length` checks for better performance. See `sort.ts` for the optimized pattern - it eliminates expensive runtime type inspection in favor of fast argument counting.
	**Performance optimization (planned):** Activities will be migrated from `isSource()` runtime checks to `arguments.length` checks for better performance. This optimization is planned but not yet implemented in `sort.ts` or other activities.

[Copilot]

The claim that this pattern appears in sort.ts, median.ts, and quartile.ts is inaccurate. The actual implementations in these files do not use .slice() before .sort(). For example, sort.ts uses (Array.isArray(source) ? source : [...source]).sort(compareFn) which mutates the array when source is an array, and median.ts uses values.sort((l, r) => l - r) which mutates the internal values array directly.

Suggested change

	**Always use `.slice()` before `.sort()` to avoid mutating input arrays:**
	```typescript
	const arr = Array.isArray(source) ? source.slice() : [...source];
	yield* arr.sort(compareFn);
	```
	This pattern appears in `sort.ts`, `median.ts`, `quartile.ts`.
	**To avoid mutating input arrays when sorting, use `.slice()` or spread syntax to create a copy before calling `.sort()`.**
	```typescript
	// Recommended pattern:
	const arr = Array.isArray(source) ? source.slice() : [...source];
	yield* arr.sort(compareFn);

Note: The current implementations in sort.ts, median.ts, and quartile.ts do not consistently use this pattern. For example, sort.ts uses (Array.isArray(source) ? source : [...source]).sort(compareFn), which mutates the array if source is an array, and median.ts uses values.sort((l, r) => l - r), which mutates the internal array directly. Consider updating these implementations to prevent unintended array mutation.

[Copilot]

The referenced test files tests/pipe.spec.ts and tests/pipe.bench.ts do not exist. Pipeline composition tests are actually in tests/chain.spec.ts. There is no benchmark file for pipe functionality in the tests directory.

Suggested change

	- `tests/pipe.spec.ts` and `tests/pipe.bench.ts` test pipeline composition
	- `tests/chain.spec.ts` tests pipeline composition

[Copilot]

Pull Request Overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

The npm run bench command does not exist in package.json, and there is no tests/pipe.bench.ts file in the repository. This documentation is inaccurate.

Suggested change

	- `npm run bench` - Performance benchmarks (see `tests/pipe.bench.ts`)
	**Test Structure:**
	- Each activity/observer has a matching `.spec.ts` file in `tests/`
	- `tests/pipe.spec.ts` and `tests/pipe.bench.ts` test pipeline composition
	<!-- Benchmark command and pipe.bench.ts file are not present. Remove this line. -->
	**Test Structure:**
	- Each activity/observer has a matching `.spec.ts` file in `tests/`
	- `tests/pipe.spec.ts` tests pipeline composition

[Copilot]

Spelling error: "indecies" should be "indices".

[Copilot]

The script name npm run test-vitest does not exist in package.json. The available test scripts are npm run test, npm run test-browser, and npm run test-node.

Suggested change

	- `npm run test-vitest` - Vitest only (dual environment: node + chromium)
	- `npm run test-browser` - Runs Vitest tests in a browser environment
	- `npm run test-node` - Runs Vitest tests in a Node.js environment