refactor: modularize the vim engine into src/vim/

[oribarilan]

Splits the monolithic src/vim.ts (656 lines) into a focused src/vim/ module tree behind a thin barrel, with tests split to mirror the layout. Rebased on v0.16.0.

What changed

• Module split: src/vim.ts becomes src/vim/ with one concern per file: types, text, tables, util, state, insert, normal, visual, plus an index.ts barrel that re-exports only the public surface. Handlers stay pure; nothing under src/vim/ touches the plugin api.
• Pending state: consolidated the two pending-state fields into a single discriminated union. This also fixes a bug where dgg and drx left a dangling delete operator that corrupted the next motion.
• Tests: test/vim.test.ts split into per-module suites under test/vim/, plus a new test/integration.test.ts for the full pipeline.
• Docs: added the modularization and text-objects design notes under docs/plans/, and updated AGENTS.md to describe the new layout.

Behavior

No user-facing behavior change beyond the dangling-operator fix. The :w/:write feature from #60 is untouched (it lives in src/index.ts, outside the engine).

Verification

just check passes locally: lint clean (24 files), 209 tests, 0 fail.