ROADMAP: write up the remaining performance work

Five items in dependency order, each sized to land on its own. The design
decisions already settled are recorded so they are not re-derived, and so
are the ordering constraints, which are the part that actually matters --
notably that the _id index must follow the tree, because it updates on
every insert and against a sorted array that is only affordable while ids
happen to append at the end.

Also carries the ground rules the earlier work established: A/B on one
harness rather than trusting a model, mutation-check tests that guard an
invariant, and the two absolutes (an index may only over-approximate; the
database must always open).

Each item names the traps found while investigating it -- deletion being
where B+trees go wrong, listIndexes possibly noticing a real _id_ index,
hashing compressed rather than uncompressed bytes, the fabricated
Document values that break when Document changes meaning, and the
durability guarantee that quietly weakens under group commit.

README.md

@@ -216,6 +216,9 @@ above is recorded in `tests/e2e/results/phase1.txt`.
 
 ### What is left (highest impact first)
 
+Each is written up with its design decisions, ordering constraints and
+traps in [ROADMAP.md](ROADMAP.md).
+
 1. **Compress the log** — the largest remaining gap (×11). Payloads are
    stored raw. A block-framed format with an LZ4 block codec would shrink
    highly compressible workloads massively; note Zig 0.16 ships zstd