v1.0: refresh README benchmarks + add a cross-order benchmark table

[espg]

🤖 from Claude

Sub-issue of #48 (v1.0 sweep), Tier-2.

Scope

The README benchmark numbers are stale (pre packed-u64 / pre order-29). Refresh them and add the cross-order dimension espg asked for:

• Re-run the existing benches (benchmarks/, bench_matrix.py) on current main and update the README figures.
• Add a cross-order benchmark table (encode/decode/coverage timing across representative orders, e.g. 4 / 12 / 18 / 29) so the order-29 performance story is visible.
• Keep it reproducible — wire the table to the existing bench harness rather than pasting hand-run numbers.

Acceptance criteria

README benchmark section updated; a documented command regenerates the cross-order table.

Refs #48.