fix(composer): coerce prefixed colour channels, non-finite numbers, marquee ids

Three more routes into the generated source, plus a fix to one of my own tests
that was checking the wrong branch.

Prefixed colour channels were interpolated raw
----------------------------------------------
Five tuples were built without coercion:

    p['fill_tuple']  = f"({el.get('r', 100)}, {el.get('g', 200)}, ...)"
    p['empty_tuple'] = f"({el.get('emptyR', 50)}, ...)"
    p['label_tuple'] = f"({el.get('labelR', 200)}, ...)"

so progress_bar, pips, sparkline and gauge took arbitrary expressions the same
way width/height did. Confirmed: every one of the five put __import__ into the
generated source. They now go through a new _rgb_tuple helper, which _rgb_expr
also delegates to.

The pre-existing colour test only covered r/g/b on a text element, which is why
the prefixed channels and these four types were never exercised.

Non-finite numbers escaped as a 500
-----------------------------------
json.loads accepts Infinity/-Infinity/NaN by default and Flask's get_json
passes them straight through, so a payload can hand _safe_int a non-finite
float. int(inf) raises OverflowError, which is neither ValueError nor
ComposerInputError, so it escaped both handlers and surfaced as a 500 with a
traceback rather than a 422. Verified end to end through Flask's parser.

Marquee ids reached the source as identifiers
---------------------------------------------
data_key is spliced UNQUOTED into variable names (_{{ data_key }}_text = ...)
and only '-' was normalised. A punctuated id landed in the generated source as
code. ast.parse caught it, so this was not exploitable, but the caller got an
opaque "Generated code has a syntax error" instead of being told the id was
unusable -- the same failure mode as the empty-block bug. Now restricted to
identifier characters and bounded to 64.

The line-anchor test was testing the wrong branch
-------------------------------------------------
test_line_branch_applies_the_anchor_offset searched the whole file for
"case 'line': {". getBoundingBox has one too and comes first, so the assertion
was reading the bounding-box branch: stripping the anchor offset from
_drawElement left all 11 checks green. Both line tests are now scoped to their
own function via tree-sitter, so they cannot be satisfied by the same branch.

Tests: 35 of the injection suite's checks fail against the reverted fixes; the
scoped line test fails when _drawElement's offset is removed. Full suite 4059
passed, the one failure being test_install_lowmem (pre-existing, awaiting #492).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01STMbQE4YctTacQXfbYqKuW

test/test_composer_js_contracts.py

@@ -96,18 +96,28 @@ def test_element_strokes_scale_with_scale():
 
 
 def test_line_branch_applies_the_anchor_offset():
-    text = CANVAS.read_text()
-    line_branch = text[text.index("case 'line': {"):]
+    """Scoped to _drawElement.
+
+    getBoundingBox has its own `case 'line': {` and appears first in the file,
+    so searching the whole text found *that* branch -- this assertion passed
+    with the draw branch's anchor offset removed. Verified: stripping it and
+    re-running gave 11/11 green.
+    """
+    body = _function_source(CANVAS, "_drawElement")
+    line_branch = body[body.index("case 'line': {"):]
     line_branch = line_branch[:line_branch.index("case 'divider'")]
     assert "ax - el.x0" in line_branch and "ay - el.y0" in line_branch, \
         "line drawing ignores xAnchor/yAnchor"
-    assert "el.x0 * s" not in line_branch, "line still drawn from unanchored endpoints"
+    assert "moveTo(el.x0 * s" not in line_branch, \
+        "line still drawn from unanchored endpoints"
 
 
 def test_line_bounding_box_applies_the_anchor_offset():
-    text = CANVAS.read_text()
-    i = text.index("case 'line':", text.index("getBoundingBox"))
-    box = text[i:i + 400]
+    """The companion to the above: scoped to getBoundingBox specifically, so
+    the two tests cannot both be satisfied by the same branch."""
+    body = _function_source(CANVAS, "getBoundingBox")
+    box = body[body.index("case 'line'"):]
+    box = box[:box.index("case 'divider'")]
     assert "ax - el.x0" in box, "line bounding box ignores the anchor"