fix(validate): fix overflowing text, and stop sampling frames that never render

[LeadcodeDev]

Two follow-ups that #170 and #171 made possible but deliberately left out of their own scope, since both change what validate does to a user's file.

--fix now answers ContentOverflowsBox on text

That arm did nothing, for a good reason spelled out in its own comment: growing the box, shrinking the font and shortening the copy are all legitimate and produce very different results, so picking one was not the validator's call.

style.text-autofit (#170) removed the dilemma. It states the author's intent — "this must fit" — without touching the declared box or the content, so nothing written by hand is overwritten or lost. That puts it in the same risk category as the two fixes already accepted (white-space removed, auto_scroll forced), both of which also change the render.

Scoped to text and gradient_text, the two components whose painters implement the field. Writing it anywhere else would be a no-op an author could reasonably read as a fix — worse than leaving the violation visible. A test pins that a table is left alone and that nothing is counted as applied.

--strict-anim stops sampling at freeze_at

Since #164 every render path clamps at scene.freeze_at, so any sample past it evaluates transforms at an instant the video cannot contain. --strict-anim was reporting violations that cannot happen — a false positive that blocks a correct scenario and sends a generator "fixing" something that was never wrong.

Bounding the sample list rather than clamping each timestamp afterwards also avoids generating a run of identical post-freeze samples, so a frozen scene costs less to validate rather than more.

scene_duration still reaches BuildAnimationCtx unchanged: duration-relative effects keep their real window (contract from PR #27), only the sampling ceiling moves.

The mirror test matters as much as the new one. The same fixture without a freeze must still be caught, or the bound would be silencing real overflow rather than removing an unreachable sample. Both are in.

Verification

• cargo test --workspace: 25 targets, 1037 tests, 0 failures
• cargo fmt --all --check and cargo clippy --workspace --all-targets -- -D warnings: clean

Clippy earned its keep here: it caught a duplicated #[test] attribute I had left above an existing one, which had been silently running the new test twice.