fix(puffin): guard ClearProperties after Finish

[fallintoplace]

Problem

Writer.ClearProperties in puffin could be called after Finish and silently mutate state because there was no finalization guard.

That is inconsistent with other mutation methods and weakens writer lifecycle safety.

Fix

• Change (*Writer).ClearProperties signature to return error.
• Add a finalization check:
  • when w.done == true, return errors.New("puffin: cannot clear properties: writer already finalized")
• Keep normal pre-finish behavior unchanged (w.props is cleared and call succeeds).
• Update tests to assert:
  • ClearProperties returns nil before finish
  • ClearProperties returns an error after finish
• Note: this is a small API-surface change because callers now handle an error return.

Validation

• go test ./puffin -run TestClearProperties -count=1
• go test ./puffin -run "Test(SetCreatedBy|ClearProperties|AddPropertiesAfterFinish)" -count=1

Scope

No behavior change for active writer usage; only enforces lifecycle consistency after finalization.

[tanmayrauth]

LGTM! Thanks for the PR.

[zeroshade]

LGTM. ClearProperties after Finish now returns an error and no-ops, matching the existing post-finalize guards. Good test coverage.