fix(textarea): reach trailing wrapped cursor lines

[LeulTew]

Let Down reach the reserved cursor row after an exact-width line, so navigation no longer gets stuck. Preserve wrapping and remembered display columns.

Refs #887.

Validation limits: Windows amd64, Go 1.27.1 only. Configured lint reports 27 unchanged CRLF formatting findings (none in changed files); race checks are unavailable without CGO/a C compiler. No interactive-terminal or cross-platform validation.

Root cause and observed cursor states

The downward probe stopped at len(line)-1, so it could not reach the end-of-line cursor position on the reserved wrapped row. Allowing len(line) fixes the stall without changing wrap(), LineInfo(), or horizontal-column restoration.

Deterministic Model observations, using an effective content width of 20 with prompt and line numbers disabled. The value is 20 ASCII a characters, a newline, then abcdefghijklmnop. Positions below are zero-based (logical line, rune column):

Before: (0,7) --Down--> (0,7)  --Down--> (0,7)
After:  (0,7) --Down--> (0,20) --Down--> (1,7)
        (0,7) <--Up---- (0,20) <--Up---- (1,7)

The intermediate (0,20) is the preserved cursor-only wrapped row. The corresponding real cursor (X,Y) positions are (7,0), (0,1), and (7,2). This is captured Model behavior, not an interactive-terminal screenshot or human visual review.

Regression coverage and local validation

The new key-message test fails in six cases before the fix and passes all nine afterward. It covers exact-width lines, repeated wraps, spaces, CJK/mixed-width runes, remembered display columns, Up reversibility, and buffer boundaries. Short-line, over-width, and word-wrap controls pass before and after; existing view expectations are unchanged.

Passed with session-local Go 1.27.1, GOMAXPROCS=2, and package parallelism limited to one:

go test -mod=readonly -p=1 -count=1 -run '^TestVerticalNavigationThroughSoftWraps$' -v .\textarea
go test -mod=readonly -p=1 -count=1 .\textarea
go test -mod=readonly -p=1 -count=1 .\...
go build -mod=readonly -p=1 .\...
go vet -mod=readonly -p=1 .\...

gofmt -l reports no changes for either modified file. The pinned golangci-lint v2.13.1 run used the existing config and one worker; all 27 reported files match the base and have CRLF checkout line endings. No lint exclusions, unrelated formatting changes, or dependency changes were made.

• I have read CONTRIBUTING.md.
• I have created a discussion that was approved by a maintainer (for new features). N/A: bug fix.