fix: improve runtime placeholder handling

[GordonSmith]

Fixes previous issues with update and remove

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
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• There are no similar instances of the same problem that should be addressed
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Testing:

[Copilot]

Pull Request Overview

This PR improves runtime placeholder handling by refactoring the notebook compilation and runtime management system. The changes rename the main compilation function and enhance the NotebookRuntime class with better cell lifecycle management.

• Renamed compileKit to compileNotebook and added compileCell for individual cell compilation
• Enhanced NotebookRuntime with separate add, update, and clear methods for better cell management
• Added proper error handling for non-existent cells and duplicate cell IDs

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
packages/observablehq-compiler/tests/index-notebookkit.ts	Updated imports and function calls to use renamed compilation functions
packages/observablehq-compiler/tests/index-notebookkit.js	Updated imports and function calls to use renamed compilation functions
packages/observablehq-compiler/src/kit/runtime.ts	Enhanced runtime with improved cell lifecycle management and error handling
packages/observablehq-compiler/src/kit/index.ts	Reordered exports for better organization
packages/observablehq-compiler/src/kit/compiler.ts	Refactored compilation to support individual cells and renamed main function
packages/observablehq-compiler/src/index.ts	Updated exports to include all kit functionality
packages/observablehq-compiler/src/compiler.ts	Updated function call to use renamed compilation function
packages/observablehq-compiler/index-kit.html	Uncommented CSS styles for better presentation

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[Copilot]

The void operator is being used to explicitly ignore the Promise returned by this.clear(cellId). However, this could lead to issues if the clear operation fails. Consider awaiting the operation or handling potential errors.

Suggested change

	void this.clear(cellId);
	await this.clear(cellId);

[GordonSmith]

The void is documenting that await is not needed here.

[Copilot]

The magic number 1000000 should be defined as a named constant to improve code readability and maintainability. Consider defining it as const SOURCE_ID_OFFSET = 1000000; at the top of the file.

[jeclrsg]

@GordonSmith Would approve. Copilot had that code-style nitpick about the source id offset being a CONST up at the top of the file. I only mention it because you'd 👎 the other one about the void