Docs: v0.21 design, acceptance and ledger

[raiseCatError]

What

The v0.21 design and acceptance document (docs/design/v021-terminal-intelligence.md) and a durable progress ledger (docs/development/v021-ledger.md) for the candidate tracked in #410.

• Reconciles Flaky: Fish adapter lifecycle test teardown races fish_update_completions #403, Flaky: queuedHighlight composer/queue classification is not settled after one event-loop turn #408, Inline multi-line ANSI progress redraws can accumulate duplicate transcript lines #388, Native read-only /view: files, logs, output and document previews #397, Native Markdown/text experience: reader, lightweight editor and optional providers #402, Semantic Output: adaptive overview and detail, raw-preserving folding, providers and copy modes #386, /track: periodically refresh a command's output in one live block #393 (draft Track live commands with /track (initial slice of #393) #394), Native module catalog: Starship / Oh My Posh coverage audit and prioritized ports #364, Module observability: explain visibility, provenance, collection timings and cache state #365, Expand bundled theme catalog: Ayu, Kanagawa, Everforest, Flexoki, Material & more #401 and Terminal-native action controls: reusable styles, semantic states and safe fallbacks #336 against dev at 6af5cbab.
• Records the confirmed causes of the three reliability issues:
  • Flaky: Fish adapter lifecycle test teardown races fish_update_completions #403: Fish's detached man-page completion generator outlives the test shell and races home removal. Test-fixture only, and the harness did not isolate data/cache directories.
  • Flaky: queuedHighlight composer/queue classification is not settled after one event-loop turn #408: a production race in SemanticService. Newer shell knowledge resolves in-flight lookups as unknown, so a stored queued command can keep the wrong color.
  • Inline multi-line ANSI progress redraws can accumulate duplicate transcript lines #388: the ANSI parser ignores row movement, so multi-row progress frames accumulate.
• Names the existing modules each slice reuses (Markdown renderer, Compare-style panels, /watch scheduler, presenter-derived rows, Context Engine capabilities, theme registry, action controls) and what stays deferred.

Docs only. No behavior change.

Refs #410