fix: restore menu-bar hiding on macOS 27 (#360)

[bhaveshnigam]

Fixes #360. Icon hiding stopped working entirely on macOS 27; this restores it using the existing mechanism, so there is no architectural change.

What's this PR do?

• Sizes the collapsed separator to 45% of the narrowest attached screen on macOS 27+, instead of twice the widest screen.
• Suppresses the separator's | glyph while collapsed on macOS 27+.
• macOS 26 and earlier are untouched (#available gated), including the existing widest-screen rule and 10,000pt bound.

Root cause. macOS 27 discards a status item whose length reaches half the display width, where macOS 26 and earlier clamped it. Hidden Bar inflates the separator to widestScreen * 2, which is over that limit on every Mac, so on 27 the separator was dropped outright: it displaced no icons and its own glyph vanished. That accounts for both symptoms reported in #360 — hiding doing nothing, and the stray separator some users see mid-menu-bar.

Staying under the limit is sufficient to hide everything, because macOS 27 owns the overflow: once the separator fills the trailing region, the icons it displaces move into the system's native overflow menu («) rather than merely off-screen. The narrowest screen is used rather than the widest — the inverse of the pre-27 rule — because one length is applied to the item on every display's bar and the limit is per display, so a length sized for a wide screen would be over the limit on a narrow one and hide nothing anywhere.

Measurements on macOS 27.0 (26A5388g), 2056pt display, by sweeping the separator length and capturing the MenuBarAgent window at each step:

separator length	result
600 / 900 / 1000pt	all icons left of the separator hidden
1028pt (exactly width / 2) and above	item dropped, nothing hidden

The limit is per item, so the always-hidden separator can be inflated at the same time (verified with both sections enabled).

What are the relevant Git tickets?

• Fixes Osx 27 broken hidden bar #360
• Does not affect the managed-overflow redesign tracked in Epic: managed-overflow / second-bar redesign (modern menu-bar hiding) #366; this restores the current mechanism rather than replacing it.

Screenshots (if appropriate)

// attach the four PNGs from ~/Desktop/hiddenbar-360-screenshots/

Any background context you want to provide? (if appropriate)

Per CONTRIBUTING:

• No new dependencies, and no project, entitlement, or preference changes. Four files touched: StatusBarController.swift, CHANGELOG.md, docs/ARCHITECTURE.md, docs/BACKLOG.md.
• Existing features tested on macOS 27: collapse and expand both directions, repeated toggling, auto-collapse timer, and the always-hidden section with both separators inflated. macOS 26 and earlier are #available-gated and take the original code path unchanged.
• Not verified, flagging deliberately: multi-display (the narrowest-screen rule is reasoned from the per-display limit, not measured on two monitors) and notched Macs. Verification used a locally compiled build of these sources rather than an Xcode archive, so a maintainer build is worth doing before release.

Verification note for anyone reproducing this: the menu bar cannot be captured with a plain screencapture when a fullscreen space covers it. Locate the MenuBarAgent window via CGWindowListCopyWindowInfo([.optionAll]) and capture it by ID (screencapture -x -o -l <id>), which works even when that window reports onscreen=false. Comparing PNG byte sizes across states is a cheap hidden/visible signal.