perf: add caching system for fast graph loading

Major performance improvements for graph visualization:

Cache System:
- Add cache.lua module that stores parsed links/tags per file
- Only re-parse files that have been modified (mtime check)
- Cache stored in .ideadrop-graph-cache.json in idea_dir
- Fast file scanning using vim.fs.find when available

Layout Optimizations:
- Reduce max iterations from 300 to 100
- Faster convergence with adjusted parameters
- Barnes-Hut approximation for large graphs (100+ nodes)
- Cache math functions locally for speed
- Skip distant node pairs in repulsion calculation
- Reuse visible_nodes array across iterations

New Commands:
- :IdeaGraph rebuild - Force full cache rebuild
- :IdeaGraphClearCache - Clear cache file

This makes opening the graph nearly instant for previously
scanned vaults, similar to Obsidian's behavior.

lua/ideaDrop/core/init.lua

@@ -250,17 +250,27 @@ function M.setup(user_opts)
 			graph.refresh()
 		elseif arg == "animate" then
 			graph.open({ animate = true })
+		elseif arg == "rebuild" then
+			graph.open({ force_rebuild = true })
 		else
 			graph.open()
 		end
 	end, {
 		nargs = "?",
 		complete = function()
-			return { "close", "refresh", "animate" }
+			return { "close", "refresh", "animate", "rebuild" }
 		end,
 		desc = "Open Obsidian-style graph visualization of notes and links",
 	})
 
+	vim.api.nvim_create_user_command("IdeaGraphClearCache", function()
+		local cache = require("ideaDrop.ui.graph.cache")
+		cache.clear()
+		vim.notify("🗑️ Graph cache cleared", vim.log.levels.INFO)
+	end, {
+		desc = "Clear the graph cache to force full rebuild",
+	})
+
 	vim.api.nvim_create_user_command("IdeaGraphFilter", function(opts)
 		local args = vim.split(opts.args, " ", { trimempty = true })