v0.7.0 review follow-ups: graceful RC keys, JSON stderr, docs

[peczenyj]

Follow-ups from the v0.7.0 batch code review (PRs #63–#74).

• Important — .structalignrc keys that don't map to a flag (notably the documented-as-RC-adjacent theme, plus typos) emit a confusing config: <key>: no such flag warning. Skip unknown keys silently; keep warning only for a real flag set to a bad value.
• Minor — RenderJSON writes its encode-error to the real os.Stderr, bypassing the injected stream. Return the error and print it via App.Stderr.
• Minor — Document that the JSON diff document always includes the summary block (independent of -summary).
• Minor — Document that -format=json ignores the presentation flags (-diff/-summary/-verbose/-color/-width).
• Minor — Tidy the unfinished trailing comment in internal/align/internal_test.go.