Support undercurl SGR styling

[jbetala7]

Fixes #5633.

Summary

• Parse SGR underline style subparameters for single, double, and curly underline, plus underline color (58) and reset (59).
• Store underline color in terminal cells and flat scrollback style data.
• Render curly underline decorations and use the SGR underline color when present.
• Preserve undercurl/underline color in grid string serialization and underline ref-test data.

Verification

• cargo check -p warp --lib
• cargo fmt --check
• git diff --check
• cargo test -p warp_terminal test_contains_cell_decorations
• cargo test -p warp_terminal test_push_rows_with_color

Notes

• Attempted cargo test -p warp test_parsing_undercurl_and_underline_color, but the local machine ran out of disk while compiling unrelated app test dependencies (lsp, ai, tantivy, warp_graphql) before the test could run.

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[oz-for-oss]

@jbetala7

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[jbetala7]

@cla-bot check

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[oz-for-oss]

Overview

This PR adds parsing, storage, rendering, and serialization support for SGR curly underline and underline colors.

Concerns

• Curly underline positioning appears incorrect: the new y value is later offset by the decoration height again, which moves the wave too far above the cell bottom.
• This is a user-visible terminal styling change, but the PR context does not include screenshots, a screen recording, or a successful app-level verification of the undercurl rendering. Manual testing is required for changes that can be manually tested. Please include screenshots or a screen recording that show it working end to end, or justify why manual testing is not possible.

Verdict

Found: 0 critical, 2 important, 0 suggestions

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[oz-for-oss]

⚠️ [IMPORTANT] This y value is subtracted by the decoration height again when building origin, so curly underlines render too far above the cell bottom. Use the cell bottom as the baseline for this decoration.

Suggested change

	thickness * 3.,
	cell_size.y() - thickness * 3.,
	thickness * 3.,
	cell_size.y(),

[jbetala7]

Addressed the requested undercurl positioning change in 38e2f1a3 by using cell_size.y() as the curly underline baseline, so the shared decoration origin applies the decoration-height offset once.

Verification run on the current head:

• cargo fmt --check
• git diff --check
• cargo check -p warp --lib

I do not have a reliable app-level visual screenshot from this local environment. The broader app-level verification was previously blocked locally by resource/environment constraints while compiling unrelated app test dependencies, so I am noting that limitation here rather than attaching a misleading screenshot. The renderer-level positioning concern has been corrected in code and the current head now passes the checks above.

/oz-review

[oz-for-oss]

@jbetala7

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Oz no longer requests changes for this pull request after the latest automated review.

[oz-for-oss]

Overview

This PR adds SGR parsing, cell/scrollback storage, serialization, and rendering support for curly underline and underline color.

Concerns

• No blocking correctness or security concerns found in the annotated diff.

Verdict

Found: 0 critical, 0 important, 0 suggestions

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