fix(render): trim IEEE-754 noise from displayed decimal arithmetic (#32)

`decimal` is stored as exact TEXT, but SQLite has no native decimal type,
so arithmetic/aggregation implicitly coerces it to an IEEE-754 double.
The computed result carries no playground type, so `sum(price * qty)`
rendered the double's full noise — `298.59999999999997` for `298.60` — a
confusing, off-topic float lesson for a teaching tool.

Add `format_real_display`: round REAL values to 15 significant figures
(a double's reliable precision) then take the shortest round-tripping
form, collapsing `298.59999999999997` to `298.6`. Wired into `format_cell`
(result-set / `show data` cells) only — the sole surface where the noise
appears, since it arises from arithmetic.

Every other f64->string path keeps full precision for semantic, not
cosmetic, reasons: CSV persistence stays byte-exact for round-trip;
`render_value` is a canonical identity key for the uniqueness dry-runs
(dry_run_unique, check_uniqueness_collisions), where rounding would
report collisions the exact-valued engine wouldn't; FK-key matching and
EXPLAIN-SQL literals likewise stay exact.

ADR-0005 Amendment 1; +7 tests.

docs/adr/README.md

@@ -10,7 +10,7 @@ This directory contains the project's ADRs, recorded per
 - [ADR-0002 — Database engine](0002-database-engine.md)
 - [ADR-0003 — Input modes and command dispatch](0003-input-modes-and-command-dispatch.md) — the persistent `Simple`/`Advanced` mode and the `:` one-shot escape. The **startup mode is no longer always `simple`**: it is restored from the project's stored mode and overridable with `--mode` (see **ADR-0015 Amendment 1**, issue #14). The app-command registry gains **`copy`** (ADR-0041, issue #11)
 - [ADR-0004 — Project file format](0004-project-file-format.md)
-- [ADR-0005 — Column type vocabulary](0005-column-type-vocabulary.md)
+- [ADR-0005 — Column type vocabulary](0005-column-type-vocabulary.md) — the ten-type set (`text`/`int`/`real`/`decimal`/`bool`/`date`/`datetime`/`blob`/`serial`/`shortid`), compound PKs, no true UUIDs; `decimal` stored as exact TEXT. **Amendment 1, 2026-06-12** (issue #32): SQLite has no native decimal/BCD type, so arithmetic/aggregation over a TEXT `decimal` is implicitly coerced to an IEEE-754 double and the computed (typeless) result leaked float noise (`298.59999999999997` for `298.60`); floating-point values are now rounded to **15 significant figures for display only** (`format_real_display` in `db.rs`, wired into `format_cell` — the result-set/`show data` cell formatter, the only surface where arithmetic noise surfaces) while every other f64→string path keeps full precision because the distinction is *semantic*: persistence (`csv_io::format_real`) stays byte-exact for round-trip; `render_value` is a *canonical identity key* for the uniqueness dry-runs (`dry_run_unique` ADR-0029 §5, `check_uniqueness_collisions` ADR-0017 §4.3) so rounding it would report collisions the exact-valued engine wouldn't; FK-key matching and EXPLAIN-SQL literals likewise stay exact — so stored `real`/`decimal` round-trips stay byte-exact and raw `decimal` columns render verbatim
 - [ADR-0006 — Undo snapshots and replay log](0006-undo-snapshots-and-replay-log.md) — **Accepted**. The **replay/journal half** (U3/U4) shipped via ADR-0034; the **undo/snapshot half** (U1/U2) is settled by **Amendment 1 (2026-05-24)** and **implemented 2026-05-24** (plan: `docs/plans/20260524-adr-0006-undo-snapshots.md`; ring in `src/undo.rs`, worker hook in `src/db.rs`). Amendment 1 **supersedes the original "snapshots only before destructive operations" model**: a snapshot is taken before **every** data/schema mutation (DSL + SQL) for familiar single-step (Ctrl-Z) undo — so the confirmation collapses to *naming the one command being undone* (no db-diff). Snapshot is a **hybrid whole-project copy** — database via the online backup API **plus** `project.yaml`/`data/*.csv` as files — reconciling this ADR with ADR-0015's "text is authoritative, db is derived"; undo restores all three directly. Staged before the mutation's transaction, finalised after the db commit (preserves ADR-0015 §6 commit-db-last); rolled-back ops leave no snapshot. **Persisted** ring under `.snapshots/`, **N = 50** (raised from 10), git-ignored + export-excluded + temp-cleanup-aware. `redo` supported, **redo stack discarded on new work**. **Batch ops record one undo step** (`replay` + future batch via a Begin/EndBatch worker primitive); **`import` is outside undo** (it switches projects per ADR-0015 §11, leaving the current project untouched). A **`--no-undo` CLI flag** disables snapshotting (hardware escape hatch). Adds the `backup` feature to `rusqlite`
 - [ADR-0007 — Sharing and export](0007-sharing-and-export.md)
 - [ADR-0008 — Testing approach](0008-testing-approach.md)