Refactor: var → const/let throughout the script

[w6a8w8]

Changes

Replaced every var declaration with const or let according to actual reassignment behaviour. No logic was changed.

Rules applied

• const — value is never reassigned after the initial declaration.
• let — value is reassigned at least once after declaration.

Classification

Module-level const (17):
playerHeight, enableOnLoad, scriptToggleKey, uw, JSStyleSheet, scriptShortName, scriptStyleId, scriptBodyClassId, viewingVideoClassId, topOfPageClassId, scriptHtmlSelector, videoContainerId, videoContainerPlacemarkerId, transitionProperties, transformProperties, ytwp, INIT_RETRY_MAX

Module-level let (2):
scriptBodySelector, scriptSelector — both are mutated inside the uw.spf block in initStyle() to duplicate the stylesheet for SPF pages.

Function-scoped locals — notable decisions:

• const d in buildVenderPropertyDict, _styleSidebar, _styleMasthead: Object is mutated via property assignment, but the binding itself is never reassigned. const is correct here.
• let d in _stylePlayer, _stylePlaylistBar: The binding is reassigned between two buildVenderPropertyDict() calls.
• const tree in hasYoutubeChanged: Array is mutated via tree.reverse() in-place. The binding is never reassigned, so const is correct. The stale tree = tree.reverse() assignment is also removed since Array.prototype.reverse() mutates and returns the same array; the assignment was redundant.
• let watchElement in setTheaterMode: Reassigned in the else-if branch via the assignment-in-condition pattern (else if (watchElement = document.querySelector(...))).
• const temp in initStyle SPF block: Assigned once, never read back. The original value of scriptBodySelector is not restored after the if block. This is a pre-existing quirk, not introduced here; using const makes the write-only nature visible.
• let debounceTimer in observePlayer: Reassigned on every mutation via clearTimeout/setTimeout.
• let parent in childOf: Reassigned in the while loop body.

What did not change

The only remaining var in the file is inside a CSS string value:
var(--ytd-masthead-height, var(--ytd-toolbar-height)) — untouched.