Implement text control select-all defaults

[wieslawsoltes]

Unchanged Code OSS sent an existing absolute folder path to the remote picker as /Users//private/tmp/... because native Meta+A reached WebScene but did not run the browser default that selects a text control's value. This adds the uncancelled Meta/Ctrl+A default action for eligible inputs and textareas, preserves Alt/cancellation/inert/disabled/non-text behavior, and exposes exact UTF-16 selection positions while retaining byte-safe WTF-8 storage.

Chromium oracle:

• absolute Unicode path: length 33, selection [0,33], replacement Z
• Aβ🙂Z: length 5, selected range [2,4], replacement AβRZ
• A🙂Z: caret [2,2] remains between surrogate units; range replacement with end selection mode produces code units [65,55357,82,56898,90] and caret [3,3]

The runtime stores an explicit UTF-16 suboffset for positions inside an astral scalar. setSelectionRange, selection getters/setters, text editing, and setRangeText convert through that representation; lone surrogate results round-trip through the existing WTF-8 DOM string boundary.

Validation on exact base 5a749a83, head 7b82b0a9:

• focused Release select-all gate: 10,000 bounded select/replace/cancel cycles in 0.491236s
• V8 heap: 679,944 -> 679,944 bytes; DOM nodes: 19 -> 19; peak RSS: 37,404,672 -> 43,466,752 bytes
• focused select-all, punctuation keyboard, and shortcut activation CTests passed
• generated WebIDL binding catalog regeneration/check passed
• live overlap audit: PR Direct media recascade through CSS dependencies #335 shares only the native test registry aggregation path and changes disjoint media-query registration lines; PR Add Code OSS browser and native desktop compatibility #76 is docs-only
• packaged Code OSS browser baseline already accepts the exact absolute fixture path; AppScene Make transition frame-demand test deterministic #131 remains open for rebuilt-package POSIX/~, spaces, Unicode, dot/dotdot, trailing separator, symlink, missing-path, <=2s settle, observer p95, cancellation, and memory acceptance

Closes #331
Related: SceneTech/AppScene#131