Warn when cell-renderings has duplicate names

[gordonwoodhull]

Summary

• Adds a warning when renderings attribute contains duplicate names (e.g., [dark, light, dark, light])
• Previously, duplicate names were silently dropped with only the last cell output for each name being used
• Users now get a clear warning explaining which outputs will be discarded

Fixes #13900

Test plan

• Verify warning is produced when using duplicate rendering names
• Run smoke-all test: ./run-fast-tests.sh docs/smoke-all/2026/01/16/13900.qmd

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[gordonwoodhull]

manually confirmed

(W) duplicate rendering name 'dark' in renderings; only the last cell output with each name will be used
(W) duplicate rendering name 'light' in renderings; only the last cell output with each name will be used

Sometimes I get Claude to write a haiku about the session, this one's a keeper

dark, light, dark, light plots
last one wins, the rest are lost
now a warning speaks