Implement CSS text-indent support

[devunt]

Closes #545

Summary

Adds CSS text-indent support to parley. The indent is treated as a margin on the start edge of the line box (per CSS spec), reducing available width during line breaking and offsetting content during alignment.

Design decisions

• Layout-level property, not StyleProperty — text-indent is a block-level CSS property, so it follows the same pattern as Alignment: stored in LayoutData, configured via a method on Layout.
• API mirrors Layout::align() — Layout::indent(amount, options) separates the core value from behavioral options.
• Per-line indent stored in LineData — computed once during line breaking, reused during alignment without recalculation.
• IndentOptions in layout module — since text-indent is a layout-level property (not a per-span style), IndentOptions lives in layout/mod.rs alongside Alignment, AlignmentOptions, and ContentWidths.

Features

• Positive indent — first line offset inward, available width reduced.
• Negative indent — first line protrudes beyond the start edge (overflow, matching CSS behavior where negative indent doesn't affect box size).
• each-line keyword — indent applies after every hard line break (BreakReason::Explicit), not just the first line.
• hanging keyword — inverts which lines are indented: continuation lines instead of first line(s).
• Alignment integration — works correctly with Start, Center, Right, and Justify alignments. Indent acts as a start-edge margin, and alignment operates within the remaining space.

API

layout.indent(50.0, IndentOptions::default());
layout.indent(50.0, IndentOptions { each_line: true, ..Default::default() });
layout.indent(50.0, IndentOptions { hanging: true, ..Default::default() });
layout.break_all_lines(Some(300.0));
layout.align(None, Alignment::Center, AlignmentOptions::default());

How it works

Line breaking (line_break.rs):

• Before processing each line, determines whether the line should be indented based on each_line and hanging options.
• Subtracts the indent from max_advance to reduce available width for the indented line.
• Stores the computed per-line indent in LineData::indent.
• Width calculation in Drop adds positive indent to line advance (negative indent causes overflow, not box growth).

Alignment (alignment.rs):

• For LTR text, sets offset = indent (previously always 0).
• Subtracts indent from free_space so alignment algorithms (center, end, justify) operate within the reduced space.
• RTL trailing whitespace handling is preserved.

Changes

File	Change
parley/src/layout/mod.rs	Add IndentOptions struct
parley/src/layout/data.rs	Add indent_amount/indent_options to LayoutData, indent to LineData
parley/src/layout/layout.rs	Add Layout::indent() public API
parley/src/layout/line_break.rs	Indent calculation in break_next, propagation through try_commit_line, width accounting in Drop
parley/src/layout/alignment.rs	Indent-aware offset and free space calculation
parley_tests/tests/text_indent.rs	22 test cases with snapshot verification

Test plan

• cargo check -p parley — compiles
• cargo test -p parley — 49 passed (existing unit tests unaffected)
• cargo clippy -p parley -p parley_tests — no warnings
• cargo test -p parley_tests — 125 passed (103 existing + 22 new)

New test cases

Test	What it verifies
text_indent_basic	Positive indent on wrapping text, only first line indented
text_indent_no_wrap	Single line with indent
text_indent_each_line	Indent after every hard break (\n)
text_indent_hanging	Continuation lines indented, first line not
text_indent_hanging_each_line	Combined hanging + each-line
text_indent_negative	Negative indent (first line protrudes)
text_indent_negative_hanging	Negative indent on continuation lines
text_indent_center_alignment	Indent + center alignment
text_indent_right_alignment	Indent + right alignment
text_indent_justify	Indent + justified alignment
text_indent_zero	Zero indent preserves existing behavior
text_indent_line_breaking	Indent causes additional line breaks
text_indent_rtl_basic	RTL text with positive indent
text_indent_rtl_hanging	RTL hanging indent
text_indent_rtl_each_line	RTL each-line indent
text_indent_rtl_negative	RTL negative indent
text_indent_rtl_center_alignment	RTL indent + center alignment
text_indent_rtl_justify	RTL indent + justified alignment
text_indent_cursor_geometry	Cursor position accounts for indent
text_indent_hit_testing	Hit testing in indent area and after indent
text_indent_negative_hit_testing	Hit testing in negative indent overflow area
text_indent_selection_geometry	Selection geometry accounts for indent

[nicoburns]

This mostly looks good to me. But we also need to make sure the RTL case is working correctly. Can we get tests for RTL case? And I think the fix I have suggested for the offset may be required.

We should also check that hit testing / cursor placement / selection works correctly with indented text (including text that overflows the bounds of it's box due to negative text indent).

[devunt]

Addressed review feedback:

• Simplified hanging indent logic using XOR (is_scope_line ^ hanging)
• Added a comment explaining why indent is not subtracted in the RTL initial offset (already accounted for in free_space)
• Added RTL text indent tests: basic, hanging, each-line, negative, center alignment, justify
• Added cursor geometry, hit testing, selection geometry tests for indented text (including negative indent overflow)

[nicoburns]

I have one suggestion around method naming. Otherwise this LGTM now. But I'll give other maintainers a chance to review before merging.

I've tested that hit testing / mouse selection logic works correctly in Blitz.

[nicoburns]

Could we name this set_text_indent. Because unlike the align method it doesn't actually apply the indent to the text (it just sets the style).