make each file more accessible - explain what the thing is for,
why you'd care, and what the patterns mean in context.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

- arraylist: focus on ownership (toOwnedSlice vs deinit), not "remember allocator"
- io: keep core patterns + tls quirk, add http.Client, delete history lesson
- build: just the 0.15 change and hash trick, delete boilerplate
- comptime: generalizable patterns, not zql description
- concurrency: design decisions (atomics vs mutex), not std.Thread syntax

-182 lines, mostly redundant/obvious content

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

- arraylist.md: why allocator is now passed to every method
- io.md: writergate, explicit buffers, tls reading quirk
- build.md: createModule pattern, build.zig.zon
- comptime.md: type-returning functions, inline for, reflection
- concurrency.md: thread pools, mutexes, atomics, backoff

README is now just an index with links

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>