Describe behaviour directly instead of by reference to the Go version

- platform::mode documents the on-disk mode-bit layout (bits 31..19 =
  d a l T L D p S u g c t ?, low 9 bits = permissions).
- The snapshot module documents the LEB128 / zig-zag varints and the RFC
  3339 JSON timestamps, and states that snapshots written by mapsize 1.x
  load unchanged.
- Renames: GoTime -> JsonTime, go_duration -> fmt_duration, fixture
  testdata/go.msz -> testdata/v1.2.0.msz (a snapshot written by mapsize
  1.2.0), loads_go_snapshot -> loads_1_2_snapshot.
- --log / --log-level help says they are accepted and ignored.

examples/bench.rs

@@ -1,5 +1,5 @@
-//! In-process benchmarks mirroring the Go `go test -bench` suite
-//! (internal/tui/bench_test.go, internal/treemap/treemap_test.go):
+//! In-process benchmarks of the TUI model (render, navigation, filtering)
+//! and the treemap layout:
 //!
 //!     cargo run --release --example bench
 //!