Build Product DSL ivar names once instead of per access

[tas50]

Summary

Each of the 10 generated Product DSL property methods interpolated a String and interned a Symbol on every read and every write:

instance_variable_get("@#{prop}".to_sym)

prop is fixed at the moment the method is defined, so the Symbol can be built once in the enclosing block and captured by the closure.

Measurements

Ruby 4.0.6:

	before	after	speedup	allocations
lookup + omnibus_project	893 ns	499 ns	1.79x	5.04 → 3.04
lookup + install_path (default)	771 ns	555 ns	1.39x	6.02 → 4.02
products_available_on_downloads_site	17193 ns	11846 ns	1.45x
all 10 props × 32 products	266 µs	220 µs	1.21x	1401 → 983

Proc-valued properties are unchanged, as expected — they are dominated by the Mixlib::Versioning.parse call in version_for, not by the ivar lookup.

One thing I tried and rejected

I also tried replacing version.to_sym == :latest in ProductMatrix#lookup with a direct comparison, on the theory that interning a Symbol per lookup was wasteful.

It measured worse — allocations went 3.04 → 4.02 and the call got slower. This file has no frozen_string_literal comment, so the "latest" literal allocates a String on every call, while to_sym on an already-interned String is nearly free. That change is deliberately not included here.

Verification

• 576 unit examples and the functional spec pass; chefstyle clean.
• Explicit assertions across String properties, Proc properties resolving on both sides of a version boundary, computed defaults, :not_available, a missing key, and the PACKAGE_ROUTER_ENDPOINT environment override.
• All 176 golden SHA256 digests match main.