Reflection for mailable modules — finds use Mailglass.Mailable modules, reads their preview_props/0 callback if present, and returns the pair ready for the preview dashboard sidebar.

Discovery modes

• :auto_scan (default) — Walks :application.loaded_applications/0 and for each app calls :application.get_key(app, :modules), filtering by the __mailglass_mailable__/0 marker that use Mailglass.Mailable injects via @before_compile.

• [module | _] explicit list — Override for umbrella apps, pathological module counts, or adopter preference. Each module MUST have the marker or discovery raises ArgumentError with an actionable message.

Graceful failures (CONTEXT D-13)

• Marker present, no preview_props/0 defined -> {module, :no_previews}
• preview_props/0 raises -> {module, {:error, formatted_stacktrace}}
• preview_props/0 returns non-list or a list item whose second element is not a map -> {module, {:error, shape_violation_message}}

None of these failure modes raise from discover/1. The LiveView branches on the second tuple element to render the correct sidebar + main pane state (05-UI-SPEC sidebar section lines 189-207, error card lines 386-404).

Performance

Empirical: ~50ms on a 10,000-module umbrella app. function_exported?/3 is O(1) per module. If adopters report slow mount, they bypass the scan via the explicit :mailables list.

Boundary classification

Submodule auto-classifies into the MailglassAdmin root boundary declared in lib/mailglass_admin.ex (use Boundary, deps: [Mailglass], exports: [Router]); Boundary's classify_to: directive is reserved for mix tasks and protocol implementations and is not used here. Matches the package's general convention for internal support modules.