refactor: migrate to built-in treesitter API and extract surround module

[fecet]

Summary

• Remove nvim-treesitter dependency, migrate to built-in vim.treesitter API
• Extract surround detection into surround.lua using TS node structure (anonymous children as delimiters) instead of text-based regex matching
• Fix selection expansion across injected language trees (e.g. JS inside HTML)
• Add bounds checking and coordinate conversion fixes

State Protocol: Pluggable Checkpoint

Current selection state is managed by two module-level tables:

selections[buf]  -- stack of selections (TSNode | {srow,scol,erow,ecol})
nodes_all[buf]   -- stack of TSNode references

This works but couples the state representation tightly to the expansion algorithm. With neovim/neovim@72d3a57 landing built-in vim.treesitter._select (providing an/in/]n/[n default mappings), Neovim will ship its own incremental selection with its own history/checkpoint mechanism.

To prepare for this, the next step is to define a checkpoint protocol — an interface that decouples "what to remember" from "how to remember it":

---@class wildfire.Checkpoint
---@field save fun(self, buf: integer, node: TSNode, range: table)
---@field restore fun(self, buf: integer): TSNode|table|nil
---@field reset fun(self, buf: integer)
---@field current fun(self, buf: integer): TSNode|table|nil

This allows swapping the backing implementation:

• Built-in backend — delegate to vim.treesitter._select history when available (Neovim nightly+)
• Custom backend — current stack-based approach as fallback for stable Neovim

With this separation, wildfire's unique value (surround-aware inner selection) becomes a thin layer on top of whichever checkpoint backend is active, rather than reimplementing the full node traversal + injection handling that Neovim now ships.

Test plan

• <CR> on cursor starts selection at TS node under cursor
• Repeated <CR> expands through parent nodes, selecting inner content first for surround nodes ((), {}, [], <>)
• <BS> shrinks back to previous selection
• Count prefix works (3<CR> jumps 3 levels)
• Selection crosses injected language boundaries (e.g. <script> in HTML)
• Empty surrounds ((), {}) are handled without error
• No nvim-treesitter required at runtime