Optimize surrogate decoding.

[lrhn]

Use char ^ 0xD800 <= 0x3FF to check if a char code is a lead surrogate. That avoids doing a later & 0x3FF to get rid of the top bits. Similar for tail surrogate.

This ensures that the high function gets values without high bits, which makes it smaller (it tries to get inlined, so a little smaller counts).

Also optimize that function to reduce dependency depth and try to hit base + (something < small) expressions that can optimized into a single x64 address computation.

Gives a ~7% increase on backwards traversal and 30% increase for forward traversal, based on tool/benchmark.dart compiled with dart compile exe.
Actually a small decrease in performance on web for forward iteration, and a small increase for backwards iteration, and Wasm follows Web in performance here.

(Also found a bug in the generator, which hasn't worked since it was last committed.)

Interestingly, the change makes little-to-no difference on the benchmark/benchmark.dart benchmark.
(Maybe even makes it a little slower.)

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:args	2.7.0	already published at pub.dev
package:async	2.13.1-wip	WIP (no publish necessary)
package:characters	1.4.1	ready to publish	characters-v1.4.1
package:collection	1.20.0-wip	WIP (no publish necessary)
package:convert	3.1.3-wip	WIP (no publish necessary)
package:crypto	3.0.7-wip	WIP (no publish necessary)
package:fixnum	1.2.0-wip	WIP (no publish necessary)
package:lints	6.0.1-wip	WIP (no publish necessary)
package:logging	1.3.1-wip	WIP (no publish necessary)
package:os_detect	2.0.4-wip	WIP (no publish necessary)
package:path	1.9.2-wip	WIP (no publish necessary)
package:platform	3.1.7-wip	WIP (no publish necessary)
package:typed_data	1.4.1-wip	WIP (no publish necessary)

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
characters	None	1.4.0	1.4.1	1.4.0	✔️

Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

Coverage ⚠️

File	Coverage
pkgs/characters/lib/src/characters_impl.dart	💚 90 % ⬆️ 0 %
pkgs/characters/lib/src/grapheme_clusters/breaks.dart	💚 97 % ⬆️ 0 %
pkgs/characters/lib/src/grapheme_clusters/table.dart	💚 100 %
pkgs/characters/tool/benchmark.dart	💔 Not covered
pkgs/characters/tool/bin/generate_tables.dart	💔 Not covered
pkgs/characters/tool/src/grapheme_category_loader.dart	💔 Not covered
pkgs/characters/tool/src/string_literal_writer.dart	💔 Not covered

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API leaks ✔️

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License Headers ✔️

// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

Files
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[mosuem]

LGTM - Although I don't have much knowledge of this package, and don't understand its intricacies. But what I understand makes sense to me.

[mosuem]

Why do the assignment instead of the original chunkStart + (tail & 255)?