fix(gif): stop making agents discover the API by failing

Closes mort#1522.

23% of all code_exec calls across two production runs (9 of 39) failed on
gifsmith API misuse, hitting 7 of 14 workers. The recovery was always the same
shape: another round trip spent printing gifsmith's own source — or, in one
case, running `dir(gifsmith)` — to learn what the module contains.

Three distinct causes, three fixes.

**Unguessable re-exports → `__all__`, plus the two that were missing.** The
module exported SOME matplotlib artists and not others, and the only way to find
out which was to fail: two workers wrote `from gifsmith import PathPatch` and
got an ImportError while `FancyBboxPatch` beside it worked. `PathPatch` and
`Path` are now exported, on the rule "the drawing vocabulary, whole". `__all__`
states the surface — and it also fixes what made self-service expensive:
`dir(gifsmith)` returned 24 names of which nine were incidental imports (glob,
os, shutil, subprocess, sys, traceback...), several being things a scene author
should never touch. SKILL.md now carries the same list, so the question should
not arise.

**Near-miss constructor kwargs → an error that names the miss.** `background=`
is accepted as an alias for `bg` (it is the obvious synonym, and an agent
reached for it). `force=` is rejected with the reason it was tempting —
`render(force="mp4")` really does take it, which makes it read like an
Animation-level setting. Anything else gets a difflib near-match and the valid
list. Both failures were one round trip each, and both are the kind a message
fixes for free.

**Scenes fail at render time → dry-run every scene at t=0 first.** A NameError
in scene 4 used to surface only after scenes 0-3 were fully rendered, and with
a streaming pipe open, after frames had gone into the encoder. Two workers lost
a whole render each to `RuntimeError: every scene failed`, learning one broken
name per attempt. One frame per scene now reports EVERY broken scene in a single
pass, before any scratch dir or ffmpeg pipe exists. Per-scene isolation is
preserved exactly: a scene that fails the dry run is marked and skipped rather
than re-run to fail identically, the others still ship, and only an
all-scenes-broken program raises early — which is precisely the case that was
paying for a full setup to learn nothing.

Verified against the production failures: `Animation(background=…)` now works,
`Animation(force=…)` explains itself, `Animation(backgrnd=…)` suggests
`background`, every name in `__all__` resolves, no incidental import leaks into
it, and the dry run marks exactly the broken scenes while leaving the good ones
untouched.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

gif/SKILL.md

@@ -76,9 +76,18 @@ bottom of this skill) as `{"name": "gifsmith.py", "file_id": "<that id>"}`, then
 write ONE `code_exec` call:
 
 ```python
-# gifsmith re-exports the drawing vocabulary, so this one import is enough:
+# gifsmith re-exports the drawing vocabulary, so this one import is enough.
+# The COMPLETE importable list (there is nothing else — don't guess, and don't
+# spend a call on dir(gifsmith)):
+#   Animation
+#   Arc Circle Ellipse FancyBboxPatch PathPatch Path Polygon Rectangle Wedge
+#   Image ImageDraw ImageFont          (PIL, for mode="pil")
+#   np plt                             (numpy / pyplot, for anything else)
+#   GIF_MAX_SECONDS GIF_MAX_BYTES
 from gifsmith import Animation, Circle, Ellipse, Rectangle, Polygon, Wedge, np
 anim = Animation(width=640, height=480, fps=15)     # canvas locked here
+# Animation() takes ONLY width, height, fps, dpi, bg (background= also works).
+# Everything about the output — format, size, audio — is set on render().
 
 # ---- cast: defined ONCE, called from every scene ----
 def draw_steve(ax, x, y, rage=0.0):