perf(lorem-optimum): speed up the Markov generation path

`cj/markov-join-tokens' collects tokens in a list and `mapconcat's once instead of repeated string concatenation. `cj/markov-generate' uses `push'/`nreverse' instead of repeated `append'. The Markov keys are cached as a vector so random key selection is O(1). Re-enabled the benchmark tests (the `:slow' tags were stale) and added a `cj/lipsum-title' test after byte-compilation flagged a malformed form there. `assets/liber-primus.txt' is left as-is (36 KB / 5,374 words, small enough not to need trimming). 100K-word learning now measures about 196 ms.

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