docs: record the direct-edit client and side conventions

Three of this task's defects were cross-layer wiring rather than logic, so they
belong in the spec where the next session will read them, not only in the task's
research notes.

Frontend: a new client spec covering layer ownership (baseline is never written,
preview hides rather than overwrites), why a handle carries only its id, why a
handle's position must derive from the clamped value, the ban on computing inside
a setState updater, and why per-pointermove React state is the wrong owner of
per-frame feedback.

Backend: the left/right convention now states the three places that must agree
and why a sign convention can be wrong on both sides of the wire at once; a table
of the four profile fields the geometry stage actually reads, so nobody builds UI
for interval-scoped editing again before the compiler honours it; and the
save-is-not-applying contract, including that compileFresh must be installed by
every path that sets session.area.

Also records that an identity test cannot catch an ignored editsFile, and that a
regression test is only trustworthy once it has been seen to fail.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.trellis/spec/frontend/index.md

@@ -14,6 +14,7 @@ This directory contains guidelines for frontend development. Fill in each file w
 
 | Guide | Description | Status |
 |-------|-------------|--------|
+| [Direct Edit Client](./direct-edit-client.md) | Handle/ghost/preview layer ownership, drag projection, session and save contracts | Filled |
 | [Directory Structure](./directory-structure.md) | Module organization and file layout | To fill |
 | [Component Guidelines](./component-guidelines.md) | Component patterns, props, composition | To fill |
 | [Hook Guidelines](./hook-guidelines.md) | Custom hooks, data fetching patterns | To fill |