feat(userland): vim 9.1 port

[FlareCoding]

Summary

• Ports Vim 9.1 (tiny feature set) for basic file editing in stlxterm, completing the arc the dup/faccessat syscalls, environment plumbing, and VT100 expansion built toward.
• Vendors a minimal subset of upstream tag v9.1.0000: 104 compiled sources, headers, proto/ and xdiff/, plus the cross-compile config.h, stubs, and Makefile proven in the closed Port Vim to Stellux: kernel termios/ioctl, stlxterm VT100, and Vim cross-compilation #113 port. A system vimrc ships at /etc/vim/vimrc since vimrc-less Vim boots in vi-compatible mode with escape-key recognition disabled.
• Acceptance testing surfaced four terminal gaps, fixed in stlxterm: unconsumed modifyOtherKeys and OSC sequences printed as garbage, DECCKM application cursor keys were not honored, and Backspace emitted DEL where Vim expects BS.

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Note

Medium Risk
Large vendored editor plus terminal parser/keymap changes in stlxterm affect every PTY session, though the edits are targeted at escape handling and input encoding rather than core shell I/O.

Overview
Adds Vim 9.1 (FEAT_TINY) to userland: vendored upstream sources, Stellux config.h / pathdef.c, stellux_stubs.c, static link against musl/lstlx, and vim in the apps build. Ships /etc/vim/vimrc with nocompatible and sensible backspace so Vim is usable without a user config.

stlxterm is updated so Vim (and similar TUI apps) behave correctly: Backspace sends \b instead of DEL; DECCKM (?1h/?1l) toggles application vs normal arrow-key encodings; the escape parser consumes OSC sequences and ignores CSI variants with unsupported prefixes/intermediates instead of leaking them to the screen.

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