sl0p: rename to sl0ppij behind a branding seam

Makes the fork installable and runnable beside a stock zellij: separate binary
name, config dir, cache dir, socket dir, log file and plugin URL scheme.

Everything user-visible is defined once in zellij-utils/src/branding.rs and
referenced from the ~15 places that used to hold a literal, rather than sed-ing
the tree -- upstream has 3300 commits of future rebases, and each touched
upstream line is a conflict forever. A conflict in branding.rs itself always
resolves the same way: keep our constant, take their code.

  binary        zellij            -> sl0ppij
  config dir    ~/.config/zellij  -> ~/.config/sl0ppij
  system cfg    /etc/zellij       -> /etc/sl0ppij
  cache/data    ProjectDirs(Zellij) -> ProjectDirs(foo.sl0p.sl0ppij)
  socket/tmp    /tmp/zellij-$UID  -> /tmp/sl0ppij-$UID
  log           zellij-log/zellij.log -> sl0ppij-log/sl0ppij.log
  plugin urls   zellij:tab-bar    -> sl0ppij:tab-bar (zellij: still parses)

Two things deliberately NOT renamed:

  - The cargo package stays `zellij`; only [[bin]] is renamed. Renaming the
    package would ripple through every intra-workspace dep, the plugin API and
    the 0.45.0+sl0p1 version trick for a name nobody sees.

  - The $ZELLIJ* environment variables stay, because the ecosystem (shell
    prompts, "am I in a multiplexer" checks, our own pi-ext) looks for them. We
    now export $SL0PPIJ* alongside them, and those take precedence on read.
    branding::import_compat_env() folds an inherited ZELLIJ_CONFIG_DIR into its
    SL0PPIJ_ spelling at startup, before clap reads the environment -- clap can
    only be told one variable name per argument.

Snapshots: 14 updated, all of them the plugin URL scheme or a session message.
The insta churn (assertion_line metadata, trailing newlines) that a bulk
--force-update introduces across 340+ other snapshots was reverted, so the diff
here is only what actually changed.

Tests: zellij-utils 471 passed, zellij-server 1499 passed. branding.rs carries
its own tests, including one asserting no shipped default path contains the
string "zellij".

Verified live: sl0ppij creates /tmp/sl0ppij-1000 next to an existing
/tmp/zellij-1000, and `sl0ppij list-sessions` reports none while a stock zellij
reports three running sessions.

0 files changed, 0 insertions, 0 deletions