LiveView-style ergonomics: :if/:for, @assigns sugar, Socket helpers

[GenericJam]

Imports a few authoring ergonomics from LiveView so Mob screens read the way Phoenix developers expect. This is the "feel" layer only — no DOM-specific bits (phx-*, JS commands, morphdom, streams are out of scope).

What's in

~MOB sigil (lib/mob/sigil.ex)

• @foo shorthand — inside any {...} expression, @foo rewrites to assigns.foo (attribute values, {expr} children, and control expressions alike), matching Phoenix HEEx. Nested forms like @user.name work too.
• :if={expr} — renders the element only when the expression is truthy. A falsy :if yields nil, which is dropped from the parent's children.
• :for={x <- list} — repeats the element per item and splices into the parent.
• :for + :if combined — :if becomes a comprehension filter (LiveView semantics): an element is produced only for items where the condition holds.

Implementation notes:

• A leading : is now accepted in attribute names at the parser level; split_control_attrs/1 pulls :if/:for out before props are built and raises on any other :-prefixed attr.
• :if/:for require a {expr} value (a string value raises a clear CompileError).
• wrap_child/1 now normalizes nil (dropped) alongside list/map, so control-wrapped children flatten cleanly through the existing List.flatten.
• Branching in wrap_control/3 is on attr presence, not the parsed expr — otherwise :if={false} would short-circuit and skip wrapping.

Mob.Socket (lib/mob/socket.ex)

• update/3 — apply a function to an existing assign (KeyError if absent)
• assign_new/3 — lazily set an assign only when absent

Both mirror Phoenix.LiveView.

Tests

Per the repo rule that sigil compile-time transforms are tested at the AST level, not just runtime: new coverage exercises self-closing and container elements, the :if/:for filter combo, @ expansion (including nested @user.name), empty-list :for, and compile-time errors for unknown/ string-valued control attrs.

• Full suite: 960 passed
• mix format, mix credo --strict, mix erlfmt --check src/: clean

Not included

No version bump — release timing is the maintainer's call.

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