feat(cli): render --progress reports frames on stderr when it is not a terminal

A terminal already gets a live `[1/1] frame 16/180 ... ~14.2s left` line while
`effectcraft-cli render` runs. A program that starts the CLI (FilmCraft runs it to
render a composition for a clip) reads stderr through a pipe, so it saw nothing
until the render ended.

`--progress` prints one line per update instead, at most four a second:

    progress <frames done> <frames total> <seconds elapsed> <seconds left, or - when unknown>

It has no effect on a terminal, which keeps its live line. Without the flag
nothing changes.

Documented in the usage text, the module header and docs/agents.md.

Tests: cargo fmt --check and cargo clippy --all-targets -D warnings pass; the
effectcraft-cli tests pass except render_out_is_relative_to_the_working_directory,
which already fails on Windows (it compares a canonicalize()d `\?\` path with the
path the CLI prints) and is unrelated to this change.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

docs/agents.md

@@ -667,6 +667,7 @@ effectcraft-cli render-frame main.ecproj --time 1 --max-side 640 --out f.png --j
 effectcraft-cli exec layer.newNull --bridge 9877                 # same commands, against the live app
 effectcraft-cli exec file.exportLottie '{"comp":"Main","path":"main.json","includeExpressions":true}' main.ecproj --json
 effectcraft-cli exec file.importLottie '{"path":"anim.json"}' main.ecproj --save
+effectcraft-cli render main.ecproj --comp Main --out main.mov --format prores --progress   # progress lines on stderr for a program that reads them
 ```
 
 Scripts written against After Effects' documented scripting API run with `script`: