v0.5.0: two-phase compute/store indexing, per-chunk prepare, FilterByRatio cost floor

A minor bump because one signature breaks: FilterByRatio gains a parameter.
Everything else is additive, and the fused AddFile/Reindex/AppendChunks
family behaves as before.

Breaking:

• FilterByRatio(maxRatio) becomes FilterByRatio(maxRatio, minCount)
  (9310fba, R675). A trigram is skipped only when it is both
  non-discriminating (over maxRatio) and expensive to scan (at least minCount
  postings). minCount of 0 or 1 reproduces the old behaviour exactly.

New:

• Two-phase indexing (3bb59d5, R676-R689). PrepareFile/PrepareContent/
  PrepareAppend compute chunks without a transaction, from any goroutine;
  StorePrepared/ReindexPrepared/AppendPrepared store them. The opaque
  single-use handles PreparedFile and PreparedAppend join the two. The store
  allocates fileids, derives the reindex removal set in its own transaction,
  and rejects an append whose tail moved (ErrAppendTailMoved).
• Per-chunk compute (06492ec, R690-R699). SplitFile/SplitContent/SplitAppend
  return SplitChunk{Chunk, Hash} plus FileMeta; PrepareChunk computes one
  chunk's grams, content-pure and cacheable by hash; AssembleFile/
  AssembleAppend package PreparedEntry values into the handles the store
  accepts. Positions always come from the entry's chunk, so reused grams are
  safe across files and scans.
• SplitAppend takes AppendOption and honours WithBaseLine (8362b4a, R700), so
  appended chunks get absolute line ranges. Existing four-argument callers are
  unchanged.

Also: mini-spec config moves to .minispec/config.toml.