Combines two orthogonal optimizations:
1. c_escape batching (from alpha): ASCII runs emitted as str_slice segments instead
of one str_char_at string per byte. O(N) allocs → O(K) where K = special chars.
2. scan_interp_string batching (from beta): char dispatch via str_char_code (Int)
+ clean_start tracking to flush plain runs as str_slice. Eliminates per-char
string allocations in the string-literal scanning hot path.
Result on web/src/main.el: 14.5MB -> 13.4MB peak RSS (-7.6%).
Self-hosting: PASS.
Will Anderson