server: coalesce undefined byte runs in the heads walker

The M2 fix (step by get_item_end so undefined bytes render and arbitrary
addresses land exactly) emits one row per undefined byte, which would explode
a large .bss/undefined region into millions of rows and make load_all crawl.

Collapse a run of consecutive undefined bytes into a single `db N dup(?)` row
(its end found in O(1) via next_head, which skips undefined). A single stray
undefined byte still renders normally (shows its value), so exact-address
navigation and the c/p/u workflow are unchanged: a 42-byte gap is one row at
the run start, and 'p' there auto-analyzes the function.

Verified: undefining a function yields one `db 42 dup(?)` row (not 42);
echo .text stays 5036 heads; data segments unaffected. region_define +
listing_view + test_domain[listing] all green. Needs a supervisor restart.

0 files changed, 0 insertions, 0 deletions