tecnickcom / gogen / 28873208995

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perf(random): fewer allocations and faster UUID/UID generation and formatting

Optimize the random package's hot paths and add zero-allocation hex formatting to the UID types, preserving public behavior (aside from the new Format/Byte methods), the crypto-first lock-free design, 100% test coverage, and clean golangci-lint.

- UUIDv7: read the 8 random bytes into a stack-local buffer via io.ReadFull instead of going through the RandomBytes wrapper, skipping the extra call and error-wrapping setup on the hot path (~3% faster generation, unchanged single allocation); the reader-failure fallback still routes through RandUint64 so generation never panics.
- RandString: read entropy into the not-yet-finalized tail of the result buffer and map accepted bytes in place, instead of allocating a fresh buffer on every rejection-refill pass; the default-charset case drops from 4 allocations to 2 and is ~12% faster at n=16, with identical rejection-sampling behavior.
- UID64: compute the decade year once rather than evaluating time.Year() twice (~5% faster).
- TUID128 and TUID64: add zero-allocation Format(*[N]byte) and Byte() methods mirroring UUID, and encode Hex() through a single stack buffer via uhex.Hex64UB; TUID128.Hex is ~36% faster now that the two-string concatenation is gone, while staying at one allocation.
- TUID128.String: append both base-36 halves into one stack buffer with strconv.AppendUint and convert once, replacing two FormatUint calls plus a concatenation; the output is byte-identical and allocations drop from 3 to 1 (~28% faster).
- Docs: add performance notes to UUIDv7 and comparative notes to UUID.Format covering the table-driven uhex encoding versus fmt.Sprintf and encoding/hex.
- Tests and benchmarks: add Format/Byte tests and benchmarks for both UID types plus a String equivalence check that locks in the byte-identical output.

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