docs(diff-ux): redesign Part I from scratch — the Review workspace

Replaces the chrome-conservative Part I presentation with a clean-slate
UI built purely from the research lessons (the prior draft stays in git
history as the low-risk fallback, per the new §3.12 migration note):

- two lines of chrome total: a scope bar (base ⟶ target · lens · burn-down
  gauge) and a context line that doubles as the action receipt
- no separate picker, History tab, or fixed comments column: doors become
  scope presets inside the workspace's scope-choosing state; History is
  the commits lens over a wide scope; comments live inline plus a notes
  projection of the one navigator rail (files/commits/notes, cycled)
- the stream is a magit-style section document with sticky context
  header, depth presets, persistent folds, chip-row verbs on the focused
  section (the only mutation surface, doubling as the keymap hint),
  format-only dimming, generated-file quarantine, and interactive
  unchanged-context separators
- three-key review loop: Space = mark-and-advance burn-down, Enter =
  zoom (scope → file → hunk → real buffer portal), Esc = back out;
  completion and resume states designed explicitly
- one view transient with sticky options; runnable ? help; Z undo with
  command echo for every mutation
- verb sets bound by scope target (stage / keep-reject / read-only) with
  the agent guardrails re-expressed in the new chrome

Ten new wireframes cover: reading state, scope-choosing, lens menu,
rail projections, line selection, comment compose, completion, resume,
view transient, buffer portal, and the agent reject receipt. Stale
door/section references in Parts II, phases, and rename table updated
to the scope-preset vocabulary.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01756Rh35n65zC4xHQcQAqBL

Author: claude