fix: fix selected-text dedenting in characterwise and blockwise modes

[pilgrimlyieu]

This PR is drafted with the help of Codex and I've reviewed and tested.

Fix #1452: LS_SELECT_DEDENT can delete whitespace inside a characterwise selection, and blockwise dedenting can depend on which corner started the selection. This series fixes those behaviors in two independently selectable patches.

Recommended submission: take A and B. The internal refactor R is an optional follow-up; neither correctness fix requires it. A and B can also be submitted separately if smaller PRs are preferred.

A. Preserve characterwise selected text

With ls.cut_keys mapped to <F5> in Visual mode, select the following text with gg^vjj$<F5>:

    first
      keep    gap
    last

The middle line currently becomes " keepgap". The expected dedented selection is:

{ "first", "  keep    gap", "last" }

The characterwise path uses unanchored substitutions. Its first-line branch also reconstructs text from the entire source line, which can include text beyond a single-line selection. Adding ^ to the substitutions alone would leave that second problem unresolved.

A removes only the overlap between the common indentation and each selected chunk. This handles partial first and last lines, single-line selections, and a selection consisting entirely of whitespace inside the line body. It reuses the existing minimum-indentation calculation and leaves the hook structure intact.

The production diff is 6 (+) 14 (-), confined to the characterwise branch of select.lua.

B. Normalize block selection columns before dedenting

For the same source text, gg01l<C-v>2j6l and gg07l<C-v>2j6h select the same rectangle from different corners. Pressing <F5> should produce the same dedented result. The old code assumes that the column of '< is the left edge, although the marks are ordered by buffer position and that endpoint can be on the right.

B uses the smaller of the two recorded byte columns in blockwise mode. Its production diff is 4 (+).

R. Optional internal refactor

R groups the operation's register and selection metadata into pending_yank, centralizes text conversion in dedent_selection, and leaves post_yank to read, restore, transform, and publish. This makes state ownership and cleanup easier to follow. Its production diff is 48 (+) 70 (-), primarily reorganization.

R is not necessary to fix #1452 so it can be reverted if needed. A+B pass all related tests (2 failed in master too in my local) and reported cases. The same suite passes with R applied. The existing public mappings, pre_yank/post_yank protocol, selected-text variables, and linewise TM_SELECTED_TEXT behavior are preserved.