docs: expand README with concrete details

Add dedicated sections for the data model (formula examples), views and
axes (tile mode, records mode, drill-down), the .improv file format
(annotated example), import wizard and headless scripting, and the
command/effect architecture. Link docs/design-notes.md from the "Why"
section. Update build instructions to use `nix build .`.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

README.md

@@ -17,18 +17,20 @@ attempt to bring that model to a modern terminal, with a formula language that
 references category and item names, views that can be rearranged with a single
 keystroke, and a plain-text file format you can diff in git.
 
+See [docs/design-notes.md](docs/design-notes.md) for the original product
+vision and non-goals.
+
 ## Quick start
 
 ```sh
-nix develop
-cargo build --release
-./target/release/improvise examples/demo.improv
+nix build .
+./result/bin/improvise examples/demo.improv
 ```
 
 Or import your own CSV:
 
 ```sh
-./target/release/improvise import path/to/data.csv
+./result/bin/improvise import path/to/data.csv
 ```
 
 ## Key bindings to try first
@@ -47,13 +49,12 @@ Or import your own CSV:
 
 ## Installation
 
-### From source with Nix (preferred)
+### With Nix (preferred)
 
 ```sh
-nix develop
-cargo build --release
-# optionally install to your PATH:
-cargo install --path .
+nix build .
+# or install into your profile:
+nix profile install .
 ```
 
 ### From crates.io
@@ -67,22 +68,109 @@ cargo install improvise
 See the [GitHub releases page](https://github.com/fiddlerwoaroof/improvise/releases)
 for prebuilt binaries (Linux x86_64, macOS Intel and Apple Silicon).
 
-## What's interesting about the codebase
+## The data model
 
-The data model stores every cell at the intersection of named categories and
-items — `(Region=East, Measure=Revenue)` — rather than at a grid address like
-`B3`. A view is a configuration that assigns each category to a row, column,
-page, or hidden axis, and the grid layout is computed as a pure function from
-`(Model, View)` to `GridLayout`. Transposing a pivot is just swapping the row
-and column assignments; records mode is just assigning the virtual `_Index` and
-`_Dim` categories to particular axes. The formula language references dimension
-names (`Profit = Revenue - Cost`), so formulas survive any rearrangement
-without rewriting. All user actions go through a command/effect pipeline:
-commands are pure functions that read an immutable context and return a list of
-effects, which are the only things that mutate state. The same command registry
-serves both the interactive TUI and the headless `cmd`/`script` CLI
-subcommands. Models persist to a markdown-like `.improv` format that is
-human-readable, git-diffable, and optionally gzip-compressed.
+Every cell lives at the intersection of named categories and items —
+`(Region=East, Measure=Revenue)` — not at a grid address like `B3`. A model
+can have up to 12 categories, each with an ordered list of items that can be
+organized into collapsible groups (e.g. months grouped into quarters).
+
+Formulas reference dimension names, not cell addresses:
+
+```
+Profit = Revenue - Cost
+Tax = Revenue * 0.08
+Margin = IF(Revenue > 0, Profit / Revenue, 0)
+BigDeal = Sum(Revenue WHERE Region = "West")
+```
+
+The formula language supports `+` `-` `*` `/` `^`, comparisons, `IF`, and
+aggregation functions (`Sum`, `Avg`, `Min`, `Max`, `Count`) with optional
+`WHERE` clauses. Formulas apply uniformly across all intersections — no
+copying, no dragging, no `$A$1` anchoring.
+
+## Views and axes
+
+A view assigns each category to one of four axes: **row**, **column**,
+**page** (slicer), or **hidden**. The grid layout is a pure function of
+`(Model, View)` → `GridLayout` — transposing is just swapping row and column
+assignments, and it happens in one keystroke (`t`). Page-axis categories act
+as filters: `[` and `]` cycle through items.
+
+Press `T` to enter tile mode, where each category appears as a tile in the
+tile bar. Move between tiles with `h`/`l`, then press `Space` to cycle axes
+or `r`/`c`/`p` to set one directly.
+
+Records mode (`R`) flips to a long-format view by assigning the virtual
+`_Index` and `_Dim` categories to row and column axes. Drill-down (`>`) on an
+aggregated cell captures a snapshot; edits accumulate in a staging area and
+commit atomically on exit.
+
+## File format
+
+Models persist to a plain-text `.improv` format that reads like markdown and
+diffs cleanly in git:
+
+```
+# Sales 2025
+
+## Category: Region
+- North
+- South
+- East [Coastal]
+- West [Coastal]
+> Coastal
+
+## Category: Measure
+- Revenue
+- Cost
+- Profit
+
+## Formulas
+- Profit = Revenue - Cost [Measure]
+
+## Data
+Region=East, Measure=Revenue = 1200
+Region=East, Measure=Cost = 800
+
+## View: Default (active)
+Region: row
+Measure: column
+format: ,.2f
+```
+
+Gzip-compressed `.improv.gz` is also supported. Legacy JSON is auto-detected
+for backward compatibility.
+
+## Import and scripting
+
+The import wizard analyzes CSV columns and proposes each as a category
+(string-valued), measure (numeric), time dimension (date-like, with optional
+year/month/quarter extraction), or skip. Multiple CSVs merge automatically
+with a synthetic "File" category.
+
+All model mutations go through a typed command registry, so the same
+operations that back the TUI work headless:
+
+```sh
+# single commands
+improvise cmd 'add-cat Region' 'add-item Region East' -f model.improv
+
+# script file (one command per line, # comments)
+improvise script setup.txt -f model.improv
+```
+
+## What's interesting about the architecture
+
+All user actions flow through a two-phase command/effect pipeline. Commands
+are pure functions: they receive an immutable `CmdContext` (model, layout,
+cursor position, mode) and return a list of effects. Effects are the only
+things that mutate app state, and each one is a small, debuggable struct.
+This means commands are testable without a terminal, effects can be logged or
+replayed, and the 40+ commands and 50+ effect types are all polymorphic —
+dispatched through trait objects and a registry, not a central match block.
+The keybinding system gives each of the 14 modes its own keymap, with
+Emacs-style prefix keys for multi-stroke sequences.
 
 ## Expectations