fix(indent): don't pull deeper indent onto blank line above unindented sibling

[sinelaw]

Summary

Fixes the corner case reported on issue #1425 after the original fix shipped.

Pressing Enter on an empty line that sits between a deeply-indented block above and an unindented line below was auto-indenting the new line to the previous block's indent — even though the very next non-empty line is already at column 0:

    line1
    line2
        line3
        line4
|                    ← cursor here, press Enter
unindented line

Before this PR the new line was indented to column 8 (matching line4); after this PR it stays at column 0.

Why the previous fix didn't catch this

The original #1425 fix (indent_for_cursor_in_leading_ws) only fires when the cursor is on a non-empty line whose leading whitespace would otherwise be displaced. On a fully empty line the heuristic in calculate_indent_pattern looked only backwards for a reference line and copied its indent (or +tab_size if the previous line ended with {/[/(/:). It never consulted what comes after the cursor, so the deeper context above always won.

Fix

Add a forward look-ahead in the empty-line branch of calculate_indent_pattern: find the first non-empty line at or after the cursor and, if its first non-whitespace byte is not a closing delimiter (}, ], )) and its indent is strictly smaller than the previous-line reference, use the smaller indent instead.

Closing delimiters are excluded so that the existing test_indent_on_empty_line_uses_reference case still indents into the function body:

fn main() {
    let x = 1;
|                    ← still indents to 4 (next non-empty is `}`, ignored)
}

Test plan

• New regression test test_enter_on_blank_line_above_unindented_uses_following_indent fails (returns 8) before the fix, passes after
• All 33 existing primitives::indent tests still pass — including test_indent_on_empty_line_uses_reference (the inside-a-block case), test_indent_after_empty_line_incomplete_syntax, and the original Auto Indent is too diligent #1425 regression test_enter_at_start_of_unindented_line_after_blank_does_not_indent
• Full cargo test -p fresh-editor --lib passes (2428 tests)
• cargo fmt --all -- --check clean
• cargo clippy -p fresh-editor --lib --no-deps introduces no new warnings on indent.rs
• Manual validation in tmux with the actual fresh binary:
  • Plain text reproducer (the user's exact case): cursor lands at Ln N+1, Col 1 (not Col 9) ✓
  • Rust file fn main() { let x = 1; <empty> }: pressing Enter on the empty line still indents to Col 5 (4-space body indent) ✓
  • Original Auto Indent is too diligent #1425 case (Enter at start of unindented line after blank): unindented line is not displaced ✓

https://claude.ai/code/session_01Rf7NU26qRrBim3DPt12eqw

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