[nanox333]

Part of #1743, the third of three parts. The first is #1758, which closes the issue.

Draft: stacked on #1758. This branch contains #1758's five commits, so until #1758 merges the diff below also shows them. This PR's own changes are the last two commits (10 files, +834/-27). I will rebase it and mark it ready for review once #1758 has merged. It does not depend on the second part.

What was wrong

Two queue items that resolve to one output (a comp added twice, a duplicated item) replace each other's file inside one job. Nothing is on disk when the job starts, so the existing-file check said nothing.

What changes

• A default output name that another queue item writes (any output module or segment) is numbered when the item is added: Main.mp4, Main_1.mp4 (renderQueue.add, render.preRender, comp.saveFrameAs {queue}, file.createProxy). Names a caller gave and names already stored in items are never changed. Sequences are compared by the frames they write, so two stills of one comp at different times keep their names.
• Outputs of one job that still name the same file (explicit names, renderQueue.duplicate, output modules, segments) are listed by renderQueue.checkOutputs under duplicates, and the whole render is refused before anything is written, naming both items. overwrite does not apply: it is for files that exist, and here the render would replace its own output. Sequences clash only on frames both write; a later item with Skip Existing Files leaves the earlier item's frames alone; items refused for footage take no part.
• The Replace / Cancel prompt stays closed in that case (the refusal shows in the status bar), and effectcraft-cli render no longer asks for --overwrite first.

Behaviour changes

A job that used to render two items over one file now refuses before writing anything. Default names change only for new items that would collide.

Testing

• tests_render_overwrite and output_guard unit tests, ui_render_overwrite (two_items_writing_one_file_are_refused_without_asking_to_replace) and a CLI test. They were written to fail without the change; I have not re-run them without it since the rebase onto Fix #1743: protect render outputs from replacing project sources #1758.
• Passing locally on top of Fix #1743: protect render outputs from replacing project sources #1758 (Windows 11, after a clean rebuild of the workspace crates): cargo fmt --check; cargo test for effectcraft-engine (lib, 700 passed), -project, -host (lib), -automation (45 + 8), -cli (cli.rs, 23 passed) and -ui-egui (lib i18n and overwrite, ui_render_overwrite 8 tests, ui_render_queue, ui_agent_audit); cargo clippy -D warnings on the engine's library and on project, host, ui-egui, automation, script and cli (all targets); cargo xtask layers; cargo check --target wasm32-unknown-unknown for project, engine and ui-egui.
• Full cargo xtask ci gate not run; quick checks only.

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