fix(storage): reject flushing lazy source step

[SunnyHaze]

Summary

Prevent LazyFileStorage.flush_step() from flushing step 0 or negative steps.

Step 0 represents the original input source rather than a writable cache output. Explicitly flushing it could serialize the source using cache_type and replace the original file, potentially changing its format or contents.

Changes

• Reject flush_step(step) when step <= 0 with a detailed ValueError.
• Explain in the error message:
  • why step 0 cannot be flushed;
  • how flushing it could damage the source file;
  • that only positive output steps buffered by write() are valid.
• Add a regression test confirming that:
  • flush_step(0) raises the expected error;
  • the original source file remains byte-for-byte unchanged.
• Keep flushing behavior for positive output steps unchanged.

Context

PR #527 protected source files from automatic flush_all() calls by limiting them to dirty output steps. This follow-up closes the remaining explicit flush_step(0) path without expanding into a broader storage lifecycle refactor.