fix(input): preserve alt-prefixed control keys

[ogulcancelik]

Summary

• decode an Alt-prefixed legacy byte by composing Alt with the inner legacy key
• preserve Ctrl+Alt+F through Kitty-enabled panes as CSI 102;7u
• add parser roundtrip and raw-input-to-pane regression coverage

Validation

• just check

Refs #2514

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 2f57bba0-cde4-494d-bf1e-ae17e6c884c0

📥 Commits

Reviewing files that changed from the base of the PR and between 2baebb5 and 5924f9e.

📒 Files selected for processing (3)

• docs/next/CHANGELOG.md
• src/input/parse.rs
• src/pane/terminal.rs

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📝 Walkthrough

Walkthrough

The input parser now preserves CONTROL and SHIFT when decoding legacy Alt-prefixed sequences. Regression tests cover round-trip parsing and Kitty pane encoding for Ctrl-Alt-F. The changelog records restored Fish keybinding support.

Changes

Ctrl-Alt key handling

Layer / File(s)	Summary
Legacy modifier parsing src/input/parse.rs	Legacy Alt-prefixed characters now reuse legacy parsing and add ALT to existing modifiers. Tests verify Ctrl-Alt-F parsing and byte round-tripping.
Kitty pane encoding src/pane/terminal.rs, docs/next/CHANGELOG.md	Pane encoding now has coverage for the ESC [102;7u Ctrl-Alt-F sequence. The changelog records restored Fish Ctrl-Alt keybinding support.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• herdrdev/herdr#2165: Both changes update legacy control-byte parsing and tests.
• herdrdev/herdr#2259: Both changes update pane terminal input encoding tests for modifier preservation.
• herdrdev/herdr#2378: Both changes update Kitty and legacy key decoding logic and tests.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the main change: preserving Alt-prefixed control keys during input parsing.
Description check	✅ Passed	The description directly explains the input fix, Kitty pane behavior, regression coverage, and validation command.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch issue/2514-fish-alt-bindings

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