add custom element api proposal doc

docs/custom-element-api.md

@@ -0,0 +1,51 @@
+# Custom Element API: Two patterns, one goal
+
+## Pattern 1: Named builder (current, preferred)
+
+A tag builder with auto-attached `Media`, created via `custom_element_builder()`:
+
+```python
+# definition (custom_elements.py)
+SessionTimestampButtons = custom_element_builder("session-timestamp-buttons")
+
+# usage (session.py)
+SessionTimestampButtons(class_="form-row-button-group", hx_boost="false")[
+    Button(data_target="timestamp_start", data_type="now", size="xs")["Set to now"],
+    Button(data_target="timestamp_start", data_type="toggle", size="xs")["Toggle text"],
+]
+```
+
+**Pros:** explicit dependency, visible import, fails loudly if builder deleted  
+**Cons:** one line of ceremony per element
+
+## Pattern 2: Element + registry (proposed, not implemented)
+
+A global `CUSTOM_ELEMENT_MEDIA` dict in `core.py` that maps tag names to their `Media`. `register_element()` populates it automatically at import time, so `Element("session-timestamp-buttons")` silently picks up its JS dependency:
+
+```python
+# definition (custom_elements.py)
+register_element("session-timestamp-buttons", "SessionTimestampButtons", EmptyProps)
+# CUSTOM_ELEMENT_MEDIA["session-timestamp-buttons"] = Media(js=("dist/elements/...",))
+
+# usage (session.py) — no builder import needed
+Element("session-timestamp-buttons",
+    [("class", "form-row-button-group"), ("hx-boost", "false")],
+    children=[...],
+)
+```
+
+**Pros:** one universal API — `Div(...)`, `Button(...)`, `Element("custom-tag")` all same pattern  
+**Cons:** implicit dependency — deleting a `register_element()` call produces no error, just broken JS at runtime
+
+## Recommendation
+
+Start with Pattern 1 (named builders) — safe by default. Add Pattern 2 later if the ceremony becomes annoying. The two are **not mutually exclusive**: a named builder is just a thin wrapper around an `Element`; the registry can be added without changing any call sites.
+
+## Quick reference
+
+| Want | Write |
+|------|-------|
+| Plain HTML tag | `Div(class_="flex")["text"]` |
+| Custom element (builder) | `SessionTimestampButtons(class_="...")[child]` |
+| Raw element | `Element("custom-tag", attributes_list, children=[...])` |
+| Builder from scratch | `custom_element_builder("tag-name")` |