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/ui/graph/init.lua

@@ -439,9 +439,9 @@ function M.open(opts)
 	-- Build graph data
 	local config = require("ideaDrop.core.config")
 	local idea_dir = vim.fn.expand(config.options.idea_dir or "")
-	vim.notify(string.format("Building graph from: %s", idea_dir), vim.log.levels.INFO)
+	vim.notify(string.format("🕸️ Loading graph from: %s", idea_dir), vim.log.levels.INFO)
 	
-	state.graph = data.build_graph()
+	state.graph = data.build_graph(opts.force_rebuild)
 
 	if #state.graph.node_list == 0 then
 		vim.notify(