app: ListingView — virtualized flat code+data listing for regions (M2)

The real disassembly-listing view. ListingView is a line-virtualized
ScrollView (reusing ColumnCursor/SearchMixin/NavMixin) backed by
ListingModel, rendering code, data (db/dw/dd, strings, jump tables) and
undefined bytes interleaved with per-kind styling. Non-function regions now
open in it instead of the M0 DisasmModel stopgap.

Integration (IdaTui): a new `listing` active mode. _open_entry routes region
entries to ListingView; _show_active/compose add it; follow/xrefs/comment and
the c/p/u edit verbs handle it (define_func upgrades a region straight to the
function view); backslash reaches hex and returns via _code_mode (so leaving
hex from a region lands back on the listing, not a func view); Tab explains
there's no pseudocode for a region. rpc._active_widget learns `listing`.

Server fix (the important one): the `heads` walker now steps by get_item_end
instead of next_head. next_head SKIPS undefined bytes, so after undefining a
function its start address wasn't even a listing line and the cursor snapped to
the next defined item; stepping by item-end renders undefined bytes as `db ?`
lines (IDA-accurate) and makes navigation land exactly on any address —
essential for "go to unmarked bytes and hit c". Needs a supervisor restart.

Tests: region_define rewritten to assert the ListingView path + segment-wide
listing + p-upgrade; new listing_view scenario (data-segment listing renders
data heads, cursor reports head ea, backslash->hex->back). Pilot 115 pass / 1
pre-existing flaky (filter); rpc_smoke 29/0; test_domain 27/0.

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