refactor: generic unbounded cache

[janishorsts]

🛠️ Benefits, Issues Resolved & DX Impact

• Benefits: Type-safe generic caching (unbounded.Cache[K comparable, V any]), zero-allocation lookup on cache hit, and concurrent value construction coalescing.
• Issues Eliminated: Refactored legacy non-generic synccache implementation, eliminating heap boxing and runtime interface{} type assertion overhead.
• Developer Experience (DX) Impact: Fully type-safe cache API with compile-time checking, simplified context propagation, and complete unit test coverage.

⚡ Performance Impact (BenchmarkCache_Do_Hit)

Metric	Before (synccache)	After (unbounded.Cache)	Delta
Execution Time	48.20 ns/op	16.54 ns/op	-65.7%
Memory Allocations	16 B/op	0 B/op	-100.0%
Heap Mallocs	1 allocs/op	0 allocs/op	-100.0%

Benchmark Breakdown

• BenchmarkCache_Do_Hit: 16.54 ns/op | 0 B/op | 0 allocs/op (Sequential cache hit lookup)
• BenchmarkCache_Do_ConcurrentHits: 136.40 ns/op | 0 B/op | 0 allocs/op (12-thread parallel cache hits)

📦 Binary Size Impact

• Stripped Size (Before/After/Delta): 30.23 MB / 30.28 MB / +48.4 KB (+0.16%)

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[github-actions]

➖ Are we earthbuild yet?

No change in "earthly" occurrences

📈 Overall Progress

Branch	Total Count
main	4753
This PR	4753
Difference	+0

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Keep up the great work migrating from Earthly to Earthbuild! 🚀

💡 Tips for finding more occurrences

Run locally to see detailed breakdown:

./.github/scripts/count-earthly.sh

Note that the goal is not to reach 0.
There is anticipated to be at least some occurences of earthly in the source code due to backwards compatibility with config files and language constructs.

[gilescope]

@janishorsts have a look - it seemed there were a few holes (demonstrated by failing tests). I've patched them with claude's help and also switched from using sleeps to something more event based. (I try and avoid sleeps in tests wherever possible)

[gilescope]

Feel free to roll back changes you don't like / reimplement!