Consider a selection background tint for HC Dark lists (list.inactiveSelectionBackground is null in hcDark

Context

While fixing the invisible active console session tab in Dark High Contrast
(#15405), we noticed an asymmetry in the upstream list color defaults
(src/vs/platform/theme/common/colors/listColors.ts):

export const listInactiveSelectionBackground = registerColor('list.inactiveSelectionBackground',
	{ dark: '#37373D', light: '#E4E6F1', hcDark: null, hcLight: Color.fromHex('#0F4A85').transparent(0.1) },

HC Light gets a subtle 10% tint for inactive selection; HC Dark gets nothing.
Any component that (reasonably) uses list.inactiveSelectionBackground as a
selection cue is invisible in HC Dark unless it also implements the outline
convention. #15405 fixed the console session tabs with a contrastActiveBorder
outline, but the question of whether HC Dark should also get a background tint
was deferred to this issue (raised in review).

Why this wasn't just fixed in #15405

The null appears deliberate rather than an oversight:

It has been hcDark: null since before Positron forked (already null in our
2022-05-24 history import) and is unchanged upstream since.
It was left null in the very edit that introduced hcLight with a tint --
the hc -> hcDark/hcLight split touched this exact registration, so the
author chose null for HC Dark while giving HC Light a value.
HC Dark's design language conveys list state with outlines only
(contrastActiveBorder / list.focusOutline); HC Light was designed later
with a tint system layered on top of outlines.
Changing the default would also:

Affect every list/tree in the workbench for HC Dark users (explorer, search,
quick picks, ...), not one component -- it's a theme-wide design decision.
Require editing an upstream file (listColors.ts), a permanent
merge-conflict liability for something upstream arguably owns.

Options

1. Do nothing -- keep the outline-only convention for HC Dark; components
   must implement outlines (as #15405 does). Users who want a tint can set:

"workbench.colorCustomizations": {
    "[Dark High Contrast]": {
        "list.inactiveSelectionBackground": "#ffffff1a"
    }
}

2. Propose the change upstream (microsoft/vscode) -- e.g.
   hcDark: Color.white.transparent(0.1) to mirror HC Light -- and inherit it
   through a future merge. Keeps us aligned and puts the accessibility design
   question in front of the theme owners.

3. Positron-local override -- set the value in our fork. Fastest, but the
   merge-liability and design-ownership concerns above apply.
   Recommendation: decide with design/accessibility input; if we want the tint,
   option 2 first.

Start with src/vs/platform/theme/common/colors/listColors.ts and review the related console session tab work in #15405. Consult the design and accessibility owners on whether HC Dark should keep outline-only selection or receive a tint, then document the decision and whether the change belongs upstream, locally, or nowhere.