feat(viewer): preserve last successful result on null/error

[laurigates]

Summary

When a query returns only null or fails (parse/run error), the viewer now keeps showing the most recent successful result instead of resetting the pane to the whole input document. This keeps the view stable so you retain context while fixing the filter (#103).

Before any query has succeeded, it still falls back to the original input. The guide pane keeps reporting the null/error exactly as before.

Behavior

Step	Before	After
.foo (valid)	shows .foo	shows .foo (remembered as last-good)
.foo.bad (null)	resets to whole document	keeps showing .foo
.foo | (error)	resets to whole document	keeps showing .foo
fix → .foo.bar	shows .foo.bar	shows .foo.bar (new last-good)

Implementation

• JsonViewer gains a last_good: Vec<Value> holding the most recent non-null successful result (empty until the first success).
• Success (non-null) updates last_good and renders it; null/error render last_good, or the original input if nothing has succeeded yet (show_last_good).
• An empty result is treated like null (the existing all(null) check is vacuously true for empty), so a filter that produces nothing also preserves the view.

The issue lists a few methodologies; this implements "result of the most recent successful query", which best matches keeping context. Easy to switch to always-original-input if you'd prefer.

Closes #103

[laurigates]

Pushed a follow-up that handles the empty-filter case for the preserved view.

Previously, clearing the editor to empty was indistinguishable from a half-typed filter: jaq rejects an empty program as invalid, so it fell through to show_last_good() — which either left the stale last result on screen or, on a fresh view, silently re-displayed the entire input document with no explanation. Clearing the filter looked like the view "wouldn't reset."

Now an empty (or whitespace-only) query is treated explicitly as "no filter": last_good is reset and the view points at the full input fresh, with a neutral no filter — showing full input status hint. Non-empty incomplete filters (.fo, .a |) still preserve the last good result to avoid flicker while typing, exactly as before.

[laurigates]

Related PRs (no strict merge order) — #120, #121, and #122 are a small series of viewer / status-line improvements that independently edit the same code (refresh_view_with_query in src/json_viewer.rs, and src/guide.rs). They're independent in intent but overlap textually, so whichever merges first, the others will need a quick rebase — no specific order is required.