Read and compile each script fragment only once

[tas50]

Stacked on #435 (Lazily load erb and ostruct) so the two do not conflict textually — they touch the same method. The first commit here is #435; review only the second. Happy to rebase onto main if you would rather take this one first or instead.

Summary

Generator::Base.get_script re-read every script fragment from disk on every call, and re-compiled every erb template from source on every call. install_sh makes 8 such calls, so generating one script did 8 File.reads and 3 ERB compiles.

Profiling a single template on Ruby 4.0.6:

stage	time	allocations
File.read	32 µs	3
ERB.new (compile)	94 µs	150
ERB#result (render)	81 µs

Reading and compiling together were ~58% of the total. The fragments ship inside the gem and do not change while the process is running, so they are now read — and compiled — once per path and reused.

Measurements

	before	after	speedup	allocations
install_sh	946 µs	274 µs	3.45x	1041 → 574
install_ps1	872 µs	288 µs	3.03x	928 → 541
Generator#install_command	818 µs	242 µs	3.38x
detect_platform_sh	36 µs	1.9 µs	19.4x	15 → 11

This matters most for anything generating install scripts repeatedly — a service answering install.sh requests pays the full compile on every request today.

Two details worth calling out

• Plain fragments are cached as Strings but returned with dup. The old code handed back a fresh String from File.read each time, so a caller is free to mutate the result. Returning the shared cached String would silently break that. There is an explicit test for it.
• The template branch tests script.is_a?(String), not is_a?(ERB). With erb lazily required, referencing the ERB constant when the first script requested is a plain fragment would raise NameError. My first draft had exactly that bug.

Verification

• 576 unit examples and the functional spec pass; chefstyle clean.
• A dedicated cache-safety check confirms: renders with different contexts do not bleed into each other, repeat renders with the same context are stable, user_agent_string is recomputed per render, mutating a returned plain script does not poison the cache, and the sh and ps1 generators do not share cache entries.
• All 176 golden SHA256 digests match main — which matters here because CI feeds the generated scripts to ShellCheck and PSScriptAnalyzer.