[audit] Guard native LSP completion when blink.cmp active; drop dead <0.11 compat

[stanfish06]

What

Two mechanical fixes in lua/config/plugin_config.lua.

1. vim.lsp.completion.enable conflicts with blink.cmp (line 140)

Before:

if client and client:supports_method("textDocument/completion") then
    pcall(function()
        vim.lsp.completion.enable(true, client.id, ev.buf, { autotrigger = true })
    end)
end

After:

if not blink_ok and client and client:supports_method("textDocument/completion") then
    ...
end

Why: When blink.cmp is installed and set up, it takes over the completion pipeline and registers its own InsertCharPre/CompleteDone autocmds. Calling vim.lsp.completion.enable(autotrigger=true) on top of that registers a second set of competing autocmds, causing both systems to fire on every keystroke. The pcall wrapper only prevents crashes — it does not prevent the runtime conflict. The guard makes native completion a fallback for when blink is absent.

2. Dead Neovim <0.11 branch in LspToggle (line 152)

Before:

if vim.fn.has("nvim-0.11") == 1 then
    client:stop()
else
    vim.lsp.stop_client(client.id)
end

After:

client:stop()

Why: The entire LSP configuration already uses vim.lsp.config / vim.lsp.enable (0.11+ only APIs), so the else branch (vim.lsp.stop_client) can never be reached on any Neovim version that can load this config.

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