feat: add inline folder creation and overflow-only navigation

[exterminatorrat]

Stacked on #51 in the coordinated five-layer stack. Replace the extra folder-name row with an inline composer while retaining the newer public editor and menu-panel changes.

• Slide the existing Add folder icon left to reveal an underlined native field and confirm/cancel actions in the same row. Enter creates and Escape cancels; failed names remain available for correction.
• Show paired navigation arrows only when measured folder content exceeds the available width. Preserve scroll position and compact-Unfiled allocation during creation, and hide/disable the arrows until composition closes.
• Fade covered navigation beneath the composer and gate its pointer, keyboard, accessibility, and native drop actions. Preserve rename behavior, native text/IME handling, and reduced-motion behavior.
• Advance VERSION to 1.0.17-beta.1. Update the directly obsolete creation-motion and composed-order audits while preserving their contract checks and adding reversed-order mutation guards.

Verification

• All 195 focused native checks passed: 19 folder-feature tests, 175 integration tests, and the owning-panel popover regression.
• All 26 build-identity tests, the identity check, Debug/Release Fleck, Release fleck-agent, and whitespace/lock-preservation checks passed.
• Fresh independent GPT-6 Astra/xhigh review approved the exact final eight-file diff. Final workflow-only base reconciliation preserves every tested source/test/script/package blob.
• Production outside the navigator/helpers is byte-identical to the lower layer. No historical development branch was imported.

Native fixture evidence

Synthetic production-view fixtures with product source identical to this head, not a newly packaged final-head app. Live pointer animation, hardware IME, and exhaustive accessibility/drag QA remain open.

Fitting and overflowing folder states

GitHub CI remains a separate gate. This publication includes no new package, app launch, accepted/development-pointer advancement, or merge.