Relocate enveditor's DS2-only textarea.go additions to textarea_ds2.go

[CLHatch]

Summary

Step 1 of scoping the bubbles textarea/selection port and the .env/YAML syntax-mode extraction discussed separately -- pure file reorganization, no behavior change.

• Compared function names between DS2's enveditor/textarea.go and upstream charm.land/bubbles/v2@v2.2.0's textarea/textarea.go: 92 vs 95, with 81 shared names (DS2's fork is still structurally close to upstream).
• Moved the 10 functions that were pure DS2 additions (never modifications of existing upstream code) to textarea_ds2.go, matching that file's own stated purpose (see its header comment): TotalDisplayLines, ResetCurrentVariable, LineMetaAt, YOffset, VisualRowToLogical, blinkTick (+ blinkTickMsg/blinkTickInterval), and the four line-number-gutter StyleState methods.
• textarea.go's function set now matches upstream's by name with exactly one exception: insertRunes, corresponding to upstream's insertRunesFromUserInput but with DS2's .env line-classification logic spliced into its body. That one needs the classification logic extracted behind a hook before it can move -- not a plain relocation, tracked as a separate follow-up.

Test plan

• go build ./..., GOOS=linux go build ./..., go vet ./..., go test ./... all pass
• Pure code motion -- no logic changed, verified via comm diff of before/after function name sets against upstream

Summary by Sourcery

Enhancements:

• Relocate DS2-specific textarea functionality into textarea_ds2.go to align the base textarea implementation with upstream structure without changing behavior.

[sourcery-ai]

Reviewer's Guide

Relocates all DS2-specific textarea helpers from the shared textarea.go into textarea_ds2.go so that textarea.go’s exported/public API and helper set match upstream bubbles’ textarea implementation, without changing behavior.

File-Level Changes

Change	Details	Files
Move DS2-only StyleState line-number gutter computed-style helpers out of the shared textarea implementation into the DS2-specific extension file.	Removed computedLineNumberFocused, computedLineNumberModified, computedLineNumberModifiedFocused, and computedLineNumberBrackets from the core textarea implementation. Reintroduced these four helpers in the DS2 extension file, preserving their bodies and comments but grouping them under a DS2-specific comment block.	internal/tui/components/enveditor/textarea.go internal/tui/components/enveditor/textarea_ds2.go
Relocate DS2-specific cursor blinking scheduling and metadata helpers from textarea.go into textarea_ds2.go.	Moved blinkTickMsg type, blinkTickInterval constant, and blinkTick command helper from the core textarea file into the DS2 extension file. Kept signatures and behavior identical to preserve existing cursor blink timing and messaging semantics.	internal/tui/components/enveditor/textarea.go internal/tui/components/enveditor/textarea_ds2.go
Move DS2-only enveditor model helpers (display line counting, env var resetting, metadata and viewport helpers) into textarea_ds2.go to isolate DS2 extensions from upstream textarea.	Relocated TotalDisplayLines, ResetCurrentVariable, LineMetaAt, YOffset, and VisualRowToLogical from textarea.go to textarea_ds2.go without modification. Ensured these helpers continue to operate on Model and its internal fields identically, maintaining behavior for env editor-specific features like default variable reset and logical/visual row mapping.	internal/tui/components/enveditor/textarea.go internal/tui/components/enveditor/textarea_ds2.go

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[sourcery-ai]

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