Strip and -Os the Spinel native binary

[tobi]

Summary

Stacked on #130. Makes make native emit a smaller release binary:

• SPINEL_FLAGS ?= -O s (Spinel wants -O LEVEL as two args)
• strip after link

dist/try went from 1,038,656 (unstripped -O2) to 696,584 (−33%). .text 924 KB → 661 KB.

Hyperfine vs stripped -O2 was noise on --help / small TUI; 1000-dir scan 9.8 vs 9.4 ms. -O3 was larger and only ~0.5 ms faster on that scan.

make native-test still 397/397.

SPINEL ?= spinel
SPINEL_FLAGS ?= -O s
NATIVE = dist/try

$(NATIVE): try.rb lib/tui.rb lib/fuzzy.rb
	mkdir -p dist
	$(SPINEL) $(SPINEL_FLAGS) try.rb -o $(NATIVE)
	strip $(NATIVE)