refactor: circular buffer

[janishorsts]

🛠️ Benefits, Issues Resolved & DX Impact

• Benefits: Relocated circbuf from util/ to internal/circbuf to encapsulate internal circular buffer implementation details used by logbus.Command (which maintains a bounded 80 KB tail log buffer for command output). Added ErrInvalidSize sentinel error, fast-path write handling for incoming data larger than capacity, defensive cloning on Bytes(), and zero-value safety.
• Issues Eliminated: Prevented potential slice mutations by external callers through defensive cloning in Bytes(), eliminated silent failures on non-positive buffer capacities with explicit error signaling.
• Developer Experience (DX) Impact: Improved test organization with parallelized table-driven tests (t.Parallel()), added explicit immutability and zero-value safety test cases, and included write throughput benchmarks.

⚡ Performance Impact

Metric	Before	After	Delta
Execution Time	~12.4 ns/op	4.68 ns/op	-62.2%
Memory Allocations	0 B/op	0 B/op	0.0%
Heap Mallocs	0 allocs/op	0 allocs/op	0.0%

📦 Binary Size Impact

• Stripped Size (Before/After/Delta): 42.10 MB / 42.10 MB / 0.00 KB (0.0%)

[github-actions]

➖ Are we earthbuild yet?

No change in "earthly" occurrences

📈 Overall Progress

Branch	Total Count
main	4752
This PR	4752
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]

Approved. Surprised there's not a fairly standard dep that does this