feat(tui2): transcript scrollbar (auto-hide + drag) (#8728)

## Summary
- Add a transcript scrollbar in `tui2` using `tui-scrollbar`.
- Reserve 2 columns on the right (1 empty gap + 1 scrollbar track) and
plumb the reduced width through wrapping/selection/copy so rendering and
interactions match.
- Auto-hide the scrollbar when the transcript is pinned to the bottom
(columns remain reserved).
- Add mouse click/drag support for the scrollbar, with pointer-capture
so drags don’t fall through into transcript selection.
- Skip scrollbar hit-testing when auto-hidden to avoid an invisible
interactive region.

## Notes
- Styling is theme-aware: in light themes the thumb is darker than the
track; in dark themes it reads as an “indented” element without going
full-white.
- Pre-Ratatui 0.30 (ratatui-core split) requires a small scratch-buffer
bridge; this should simplify once we move to Ratatui 0.30.

## Testing
- `just fmt`
- `just fix -p codex-tui2 --allow-no-vcs`
- `cargo test -p codex-tui2`

codex-rs/Cargo.lock

@@ -1820,6 +1820,7 @@ dependencies = [
  "pulldown-cmark",
  "rand 0.9.2",
  "ratatui",
+ "ratatui-core",
  "ratatui-macros",
  "regex-lite",
  "reqwest",
@@ -1841,6 +1842,7 @@ dependencies = [
  "tracing-subscriber",
  "tree-sitter-bash",
  "tree-sitter-highlight",
+ "tui-scrollbar",
  "unicode-segmentation",
  "unicode-width 0.2.1",
  "url",
@@ -2005,6 +2007,20 @@ dependencies = [
  "static_assertions",
 ]
 
+[[package]]
+name = "compact_str"
+version = "0.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3fdb1325a1cece981e8a296ab8f0f9b63ae357bd0784a9faaf548cc7b480707a"
+dependencies = [
+ "castaway",
+ "cfg-if",
+ "itoa",
+ "rustversion",
+ "ryu",
+ "static_assertions",
+]
+
 [[package]]
 name = "concurrent-queue"
 version = "2.5.0"
@@ -2583,6 +2599,15 @@ version = "0.3.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fea41bba32d969b513997752735605054bc0dfa92b4c56bf1189f2e174be7a10"
 
+[[package]]
+name = "document-features"
+version = "0.2.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d4b8a88685455ed29a21542a33abd9cb6510b6b129abadabdcef0f4c55bc8f61"
+dependencies = [
+ "litrs",
+]
+
 [[package]]
 name = "dotenvy"
 version = "0.15.7"
@@ -3909,6 +3934,16 @@ dependencies = [
  "wasm-bindgen",
 ]
 
+[[package]]
+name = "kasuari"
+version = "0.4.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8fe90c1150662e858c7d5f945089b7517b0a80d8bf7ba4b1b5ffc984e7230a5b"
+dependencies = [
+ "hashbrown 0.16.0",
+ "thiserror 2.0.17",
+]
+
 [[package]]
 name = "keyring"
 version = "3.6.3"
@@ -4054,6 +4089,12 @@ version = "0.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "241eaef5fd12c88705a01fc1066c48c4b36e0dd4377dcdc7ec3942cea7a69956"
 
+[[package]]
+name = "litrs"
+version = "1.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "11d3d7f243d5c5a8b9bb5d6dd2b1602c0cb0b9db1621bafc7ed66e35ff9fe092"
+
 [[package]]
 name = "local-waker"
 version = "0.1.4"
@@ -5393,7 +5434,7 @@ source = "git+https://github.com/nornagon/ratatui?branch=nornagon-v0.29.0-patch#
 dependencies = [
  "bitflags 2.10.0",
  "cassowary",
- "compact_str",
+ "compact_str 0.8.1",
  "crossterm",
  "indoc",
  "instability",
@@ -5402,7 +5443,27 @@ dependencies = [
  "paste",
  "strum 0.26.3",
  "unicode-segmentation",
- "unicode-truncate",
+ "unicode-truncate 1.1.0",
+ "unicode-width 0.2.1",
+]
+
+[[package]]
+name = "ratatui-core"
+version = "0.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5ef8dea09a92caaf73bff7adb70b76162e5937524058a7e5bff37869cbbec293"
+dependencies = [
+ "bitflags 2.10.0",
+ "compact_str 0.9.0",
+ "hashbrown 0.16.0",
+ "indoc",
+ "itertools 0.14.0",
+ "kasuari",
+ "lru 0.16.2",
+ "strum 0.27.2",
+ "thiserror 2.0.17",
+ "unicode-segmentation",
+ "unicode-truncate 2.0.0",
  "unicode-width 0.2.1",
 ]
 
@@ -6569,6 +6630,9 @@ name = "strum"
 version = "0.27.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "af23d6f6c1a224baef9d3f61e287d2761385a5b88fdab4eb4c6f11aeb54c4bcf"
+dependencies = [
+ "strum_macros 0.27.2",
+]
 
 [[package]]
 name = "strum_macros"
@@ -7415,6 +7479,16 @@ dependencies = [
  "termcolor",
 ]
 
+[[package]]
+name = "tui-scrollbar"
+version = "0.2.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c42613099915b2e30e9f144670666e858e2538366f77742e1cf1c2f230efcacd"
+dependencies = [
+ "document-features",
+ "ratatui-core",
+]
+
 [[package]]
 name = "typenum"
 version = "1.18.0"
@@ -7482,6 +7556,17 @@ dependencies = [
  "unicode-width 0.1.14",
 ]
 
+[[package]]
+name = "unicode-truncate"
+version = "2.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8fbf03860ff438702f3910ca5f28f8dac63c1c11e7efb5012b8b175493606330"
+dependencies = [
+ "itertools 0.13.0",
+ "unicode-segmentation",
+ "unicode-width 0.2.1",
+]
+
 [[package]]
 name = "unicode-width"
 version = "0.1.14"