docs(site): rework protocol docs into Binary Protocol section (#67)

The 857-line schema page sat buried under Server and mixed every
protocol concern, the replica-to-replica wire was undocumented, and
the storage-engine write-pipeline diagram rendered unreadably while
contradicting the code.

Schema content now lives in a top-level Binary Protocol section
split by concern (framing, connection lifecycle, commands, shared
encodings, message batches), plus a new server-to-server page
written against the consensus code: command registry, frame seals,
handshake, replication, view changes, repair, state transfer, and
session forwarding. A meta-refresh stub keeps the old
/docs/server/schema/ URL alive since the static export cannot
redirect.

The write-pipeline diagram is redrawn without the back-edge cycle
and long edge label that scrambled the site's mermaid renderer, and
its claims are corrected against the server: stamping happens at
journal-append time before replication acks, buffering at prepare
time, the flush gate is a three-way OR evaluated on commit events,
and enforce_fsync does fdatasync on both the .log and .index.
Inherited errors fixed along the way: the header-kinds table was
missing code 8 (Int128); the server-stamped message time is the
flat batch base_timestamp, not base_timestamp + timestamp_delta;
and compression_algorithm is now documented as a placeholder (the
value is stored and reported, but nothing compresses payloads yet -
the message-headers examples show the client-side pattern).

Two rendering fixes ride along. The mermaid component bumped
font-size attributes after layout, overflowing node boxes sized for
the original metrics and colliding adjacent nodes; it now zooms the
whole SVG uniformly instead. And the fumadocs mobile TOC bar
([data-toc-popover]) is lifted to z-30 with an opaque background:
it was sticky at z-10 and transparent, so docs content using z-10
painted over it while scrolling under.