fix(toggles): preserve treesitter syntax state

[mehalter]

Tree-sitter toggles

• Route buffer syntax enablement and disablement through AstroCore's Tree-sitter helpers.
• Keep AstroCore's per-buffer enabled state synchronized with the active parser state.
• Record an explicit disabled state even if the parser disappears before the toggle runs.

Validation

• Passed StyLua, Selene, typos, Lua compilation, and diff checks.
• Passed the focused syntax-state regression on Neovim 0.11.0 and 0.12.4.