feat: teach defaultdict and map/filter on the canvas (curriculum phase 0c/0d)

- defaultdict(factory): dict box carrying a default factory; a missing-key
  read both returns and stores the factory value
- map(fn, it) / filter(pred, it): lazy iterator objects that hold a reference
  to both the source iterator and the function, drawn as a reference chain
- functools.reduce(fn, it): eager fold
- bare constructor names (int, list, ...) now resolve to builtin objects so
  they can be passed as factory/transform arguments
- canvas + snapshot: mapiter/filteriter edges (source + fn), defaultdict
  factory tag and entry refs
- new 'Defaults & Functional' series: 4 levels (35 total across 9 series)
- README/DESIGN updated to 35 levels / 9 series

Author: alisadeghiaghili

DESIGN.md

@@ -26,11 +26,15 @@ Supported subset (deliberately small, pedagogically sharp):
 - Function `def` / call / `return`, local frames, nested calls
 - `print`, `type`, `id`, `len`, `is`, `==`, arithmetic, comparisons
 - `import copy as cp` (simulated module) → `cp.copy` (shallow) / `cp.deepcopy`
+- `import collections as c` → `c.defaultdict(factory)` → dict box with a default-factory
+- `import functools as f` → `f.reduce(fn, it)` → eager fold
 - `iter(x)` / `next(it)` → stateful iterator object (source reference + cursor)
+- `map(fn, it)` / `filter(pred, it)` → lazy iterator object (source + function reference)
 - Plotting: `scatter(xs, ys, "title")` / `line(xs, ys, "title")` → SVG board
 - Meta: `help`, `levels`, `level`, `goal`, `hint`, `solution`, `undo`, `reset`, `golf`, `script`
 
-Out of scope v0: classes/OOP, real imports (only the simulated `copy` module), control flow
+Out of scope v0: classes/OOP, real imports (only the simulated `copy`, `collections`, `functools`
+modules), control flow
 (`if`/`for`/`while`), exceptions (except parse/runtime errors shown cleanly), comprehensions beyond
 simple cases, I/O, async, `range`/`sorted`/`min`/`max`/`abs`.
 
@@ -43,7 +47,7 @@ simulated module whose data structures are first-class heap objects on this canv
 ```
 src/engine/    simulated Python runtime (tokenizer, parser, evaluator, memory,
                session) + goal checks (checks.ts) — the TreeCompare analogue
-src/levels/    8-series / 31-level catalog + per-level goals, par, solutions
+src/levels/    9-series / 35-level catalog + per-level goals, par, solutions
 src/ui/        app shell, memory canvas, plot board, REPL, dialog, script editor
 src/style.css  design tokens + layout
 index.html     app shell entry

README.md

@@ -41,7 +41,7 @@ Type Python-subset statements. Meta-commands:
 | Command | Effect |
 |---------|--------|
 | `help` | subset + meta overview |
-| `levels` | list the 31 levels / 8 series |
+| `levels` | list the 35 levels / 9 series |
 | `level <id>` | load a level (e.g. `level first-name`) |
 | `lesson` | open the current level's lesson text |
 | `goal` / `guide` | focus the right-hand guide panel |
@@ -63,12 +63,13 @@ A deliberately small, pedagogically sharp subset — not CPython:
 - **Methods:** `.append` `.insert` `.extend` `.pop` `.remove` `.copy` `.get` `.update` `.keys` `.clear`
 - **Functions:** `def` / call / `return`, local frames, nested calls
 - **Builtins:** `print` `len` `type` `id` `str` `int` `bool` `list` `dict` `set`
-- **Modules:** `import copy as cp` — `cp.copy` (shallow) and `cp.deepcopy`
+- **Modules:** `import copy as cp` — `cp.copy` (shallow) and `cp.deepcopy`; `import collections as c` — `c.defaultdict`; `import functools as f` — `f.reduce`
 - **Iterators:** `iter(x)` and `next(it)` — a stateful iterator object you advance by hand
+- **Functional:** `map(fn, it)` / `filter(pred, it)` — lazy iterator objects; `f.reduce(fn, it)` — an eager fold
 - **Plotting:** `scatter(xs, ys, "title")` `line(xs, ys, "title")`
 - **Operators:** arithmetic, comparisons, `is`, `==`
 
-Not in v0: classes, real module imports (only the simulated `copy`), control flow (`if`/`for`/`while`), exceptions (beyond clean parse/runtime errors), comprehensions, I/O, async, `range`/`sorted`/`min`/`max`/`abs`, full CPython semantics.
+Not in v0: classes, real module imports (only the simulated `copy`, `collections`, `functools`), control flow (`if`/`for`/`while`), exceptions (beyond clean parse/runtime errors), comprehensions, I/O, async, `range`/`sorted`/`min`/`max`, full CPython semantics.
 
 A planned curriculum extends this with numpy, pandas, re, datetime and more — all drawn on the same memory canvas. See `docs/ENGINE-EXTENSION.md` and `docs/adr/002-library-modules.md`.
 
@@ -77,7 +78,7 @@ A planned curriculum extends this with numpy, pandas, re, datetime and more —
 ```
 src/engine/    simulated runtime (tokenizer, parser, evaluator, memory, session)
                + goal checks (checks.ts) — the "TreeCompare" analogue
-src/levels/    8-series / 31-level catalog + per-level goals, par, solutions
+src/levels/    9-series / 35-level catalog + per-level goals, par, solutions
 src/ui/        app shell, memory canvas, plot board, REPL, dialog, script editor
 src/style.css  design tokens + layout
 tests/         vitest unit + level-solution tests
@@ -89,7 +90,7 @@ The engine is intentionally a **teaching subset**, not CPython. Every state tran
 
 ## Levels
 
-Eight series — Names & Bindings, Aliasing & Copy, Containers in Memory, **Copy Semantics** (shallow vs deep), **Iterators & Iterables**, Mutation & Methods, Functions & Frames, Plotting & Capstone. Each of the 31 levels has a brief, optional hint, a goal (predicate or target snapshot), a statement-par for golf, and a worked solution.
+Nine series — Names & Bindings, Aliasing & Copy, Containers in Memory, **Copy Semantics** (shallow vs deep), **Iterators & Iterables**, **Defaults & Functional** (defaultdict, map, filter), Mutation & Methods, Functions & Frames, Plotting & Capstone. Each of the 35 levels has a brief, optional hint, a goal (predicate or target snapshot), a statement-par for golf, and a worked solution.
 
 ## Tech
 

src/engine/checks.ts

@@ -225,6 +225,14 @@ export const preds = {
     };
   },
 
+  /** `name` is a defaultdict (not a plain dict). */
+  isDefaultDict(name: string) {
+    return (snapshot: Snapshot): boolean => {
+      const view = slotView(name, snapshot);
+      return view?.type === 'defaultdict';
+    };
+  },
+
   /**
    * A plot has been drawn (optionally of a specific kind).
    */