server+domain: heads walker + ListingModel — flat code/data listing (M1)

The keystone for a real disassembly-listing view (unlike DisasmModel, which
is one function, code-only). Two pieces:

server/patch_server.py: inject a `heads` tool. It walks item heads over a
segment with next_head/prev_head and renders each via generate_disasm_line,
so it returns a flat listing where code, data (db/dw/dd, strings, jump
tables) and undefined bytes all appear as typed rows {ea,kind,size,text,name}.
Unlike `disasm` (code-only, bails at the first data byte) it shows the whole
segment. Address-paged: chain forward via cursor.next, page up with back=true
(returns the N heads ending before addr, in forward order, + cursor.prev).

domain.py: ListingModel — a lazily-grown, segment-scoped head index
(FunctionIndex-style forward paging via cursor.next; line index == position
in the walked list). ensure_ea() gives random access to an address (resolving
a mid-item byte to its containing head). New Head dataclass, Program.listing()
(cached per segment) + segment_bounds(); section_of() now derives from it;
bump_items() clears the listing cache too.

Verified live: heads renders strings/jump-tables/unknown correctly, forward
chaining + back-paging work; ListingModel walks echo .text (5036 heads,
~276ms) with cached windows and mid-item address resolution. tests/test_domain
gains a [listing] section (27 passed). Pilot hex/region_define/disasm_nav green.

Note: adding a server tool needs a supervisor restart (workers respawn).

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