streams: per-invocation channel, pumping mode, filter pipeline wiring

exec: LineChannel bounded ring (cap=16) with abort/eof/marker fields.
Shell 'pumping mode' assembles canonical lines from its queue into the
channel while the command runs; ^C marks the channel aborted.

Shell-side grammar: USE/TIL pre-drain into the channel before spawn;
<cmd> <path> TIL <marker> sugar drains paths until the marker line.
Interactive TIL: pump_key checks each submitted line against the marker
and closes the channel when it matches.

Exports callbacks run in the command task but touch shell-owned state:
this is safe by construction — the shell blocks in exec_external until
the command completes.  put_line feeds Pipeline::line directly;
get_line pops from the channel (yield-poll when empty).

load: drop NX from image mapping (single-privilege kernel, NX bit 63
reserved when EFER.NXE is set by OVMF); compute image span from section
RVAs not file size.

bins/count: gate binary — reads lines via get_line, reports count via
put_line.  Verified: count USE file → 6; count <path> TIL END → 2;
interactive count TIL DONE → 3; -h filter across task boundary;
^C abort.

v1 stream exports: put_line → Pipeline, get_line ← channel, log_line
→ klog append, exit → abort_current.

Accounting: print_one_line → pipe.line() returns false (head kill /
pager ^C) → the binary should wrap up promptly; accepted: binary does
not check the return value in v1 (phase 3 simplification).

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