Fix up landed PRs from review

- #178: ToUnicode ranges looked up without indexing
- #465: frame_num gaps are concealed (bounded) even when the SPS forbids them; cut streams keep playing
- #471: control request lines up to 256 MB so picture/media inserts still fit
- #473: a merge that loses its only row or column is removed, with regression tests
- #521: AutoCorrect no longer rewrites real words (cant, wont, im)
- #654: inherit_placeholder_geometry's doc comment back on its function

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

crates/engine/src/cmd/text.rs

@@ -368,8 +368,6 @@ pub const AUTOCORRECT: &[(&str, &str)] = &[
     ("dont", "don't"),
     ("doesnt", "doesn't"),
     ("didnt", "didn't"),
-    ("cant", "can't"),
-    ("wont", "won't"),
     ("isnt", "isn't"),
     ("embarass", "embarrass"),
     ("enviroment", "environment"),
@@ -379,7 +377,6 @@ pub const AUTOCORRECT: &[(&str, &str)] = &[
     ("goverment", "government"),
     ("happend", "happened"),
     ("i", "I"),
-    ("im", "I'm"),
     ("ive", "I've"),
     ("immediatly", "immediately"),
     ("independant", "independent"),