fix: preserve relative indent when pasting multi-line charwise blocks (]p/[p)

[nemanjamalesija]

Fixes #11.

The real bug

<DateField.Group>
  <Button>Select Month</Button>
</DateField.Group>
<DateField.Group>
      <Button>Select Month</Button>   // before: over-indented
    </DateField.Group>                // before: over-indented

Cause

A charwise selection (e.g. ^v...y) drops the first line's leading whitespace, but the inner lines keep their absolute indent. The ]p/[p path then shifted every line by a single delta computed from the first line — so the inner lines got the block's original base indent counted twice. (Vim's native ]p has the same flaw; smart-paste should do better.)

"Copying the whole line" worked because that's a linewise yank, which keeps the first line's indent and goes through a different path.

Fix

When a block dedents back below its first inner line (a closing tag/brace — the normal shape of a JSX element, {}, () block), the first line is an opener whose true base equals the block's minimum inner indent. Reconstruct it to that base before shifting, so the block's relative structure is preserved.

Blocks that never dedent (e.g. if x: + body, where the first line genuinely sits at column 0) keep the first line at 0 — existing behavior is unchanged.

After:

<DateField.Group>
  <Button>Select Month</Button>
</DateField.Group>
<DateField.Group>
  <Button>Select Month</Button>   // matches the original
</DateField.Group>

Tests

• indent_spec.lua: unit tests for the new rebase_charwise_block (dedenting block, non-dedenting block, single line).
• charwise_paste_spec.lua: ]p of a multi-line block from a mid-line charwise selection now matches the original structure instead of over-indenting.

All existing specs and integration checks pass.