test: add automated Neovim coverage

[lmilojevicc]

Summary

Adds an isolated mini.test suite and Ubuntu CI so compatibility and core Neovim behavior are exercised automatically from Neovim v0.10.0 through stable.

Changes

• Add a pinned mini.test harness, minimal init, and reproducible make targets.
• Add Ubuntu CI for Neovim v0.10.0 and stable, plus Bash syntax validation.
• Add 32 isolated cases covering configuration, windows, the Herdr process boundary, navigation, resizing, and public module loading.
• Harden malformed/JSON-null Herdr responses discovered by regression tests.
• Keep focus in newly created native splits while preserving Neovim's splitright / splitbelow placement behavior.

Validation

• make test passes
• make check-shell passes
• Plugin loads with no errors in a Neovim + Herdr session (HERDR_ENV=1)
• :checkhealth herdr-splits reports no failures
• Navigation (<C-h/j/k/l>) works across Neovim splits and Herdr panes
• Resizing (<M-h/j/k/l>) works across Neovim splits and Herdr panes
• Edge cases relevant to this change exercised (at_edge, count prefix, floats, auto-unzoom)
• README.md / herdr-plugin.toml updated (not required; no configuration or manifest change)

The full 32-case suite passes on Neovim v0.10.0 and v0.12.4. Manual Herdr cross-pane integration remains intentionally manual and was not run as part of this change.

Notes for review

Herdr-pane split behavior is deferred to follow-up issue #15 and is not implemented by this PR.