Fix compounding code injection on repeated runs of the same block

[nelsonlove]

Summary

handleExecution assigns the injected source back into the block's CodeBlockContext (block.srcCode = await ...injectCode(srcCode)). That context is shared between runs of the same rendered block (it's captured in the run button's click listener), so every re-run injects pre-/post-/import code on top of the previous run's already-injected source.

Reproduction: with a labeled block ({label="x"}) and a second block importing it ({import="x"}), click Run on the importing block N times without re-rendering the note → the imported code is included N times (visible as N repeated definition echoes in interactive mode, and N-fold side effects for non-idempotent imports).

A side effect of the stale mutation is that CodeInjector.parseFile can no longer match the block against the note's source on re-runs (it compares srcCode to the file's block content to find the block's own args), so the block's own import/ignore args are silently dropped from the second run onward.

Fix

Run against a shallow copy of the context: block = { ...block, srcCode: <injected> }. The shared context is never mutated; runCode's optional transform also operates on the copy.

Testing

• npm run build passes
• Manually verified: repeated runs of an importing block now produce identical output each time (previously the imported definitions accumulated once per click)

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