Implement single-dict fast-path for pure-union PrefixMatch subtrees

[frankslin]

When a dict group's entire subtree consists only of union groups and
leaf dicts (no short_circuit node anywhere), union is associative and
the tree is semantically equivalent to a single flat union of all its
leaves. A single LeafMatcher trie traversal finds the longest match
across every leaf, which equals the multi-trie result while eliminating
GroupMatcher dispatch and redundant trie walks.

Adds CanFlattenAsUnion (recursive predicate) and CollectAllLeafDicts
(leaf accumulator), wired into BuildMatcher as an early-exit fast-path
before the existing GroupMatcher construction.

[frankslin]

經測試，三種輸入長度（7.7kb 44kb 630kb）s2t 均都有穩定的 16–22% 提升，且越長的輸入優勢越明顯。
多出的 init 開銷是 ~2ms（32.3ms vs 30.4ms），同一 config 只會 build 一次，進程生命周期內後續調用直接復用。