Agent: add zmodem_send for unattended PC→device upload

[typeless]

Adds a zmodem_send agent method so a client can push a local file to the device over the session Tera Term already holds open — no file-picker, no DDE, no window relaunch. It closes the one remaining gap in hands-off ZMODEM.

Why

Device→PC download is already unattended via the built-in auto-receiver (verified this cycle: 128 KB byte-perfect). The reverse was stuck:

• Built-in auto-send stops at a GUI file-picker (ZMODEMStartSend(NULL,...) → _GetMultiFname).
• ttpmacro cannot attach to an interactively-launched window — it registers no DDE server without /D= (InitDDE guarded by strlen(TopicName)>0; the live window's cmdline has no /D=, verified), and its connect spawns a new ttermpro that fights for the COM port.

So neither existing route is unattended. zmodem_send drives Tera Term's own send engine over the existing session instead.

How

• Agent requests already run on the GUI thread (the hidden A.wnd), so be_zmodem_send calls ZMODEMStartSend inline — no cross-thread marshaling.
• The transfer is async on the protocol pump; its outcome is surfaced through a new transfer object in status — a tagged union (state: idle|active|done, with ok present only when done), fed by a hook at the single ProtoEnd completion funnel.
• Client flow: arm the device receiver (rz -b -y), call zmodem_send, poll status.transfer until done.
• Reuses the existing send-armed gate — grants nothing beyond send_bytes (reads a local file, streams it out the serial line, both already within the arm's scope).

Scope

Local (focused) window only. A foreign window's transfer is neither driveable nor observable from this process (status reads this window's cv_ProtoFlag), so cross-window ZMODEM is deferred and returns NOTALLOWED; send_bytes/text still hand off cross-window via the shm queue.

Verification — split, and stated honestly

Host-tested (ASan/UBSan, all green):

• test_zmodem_send — JSON dispatch / path+binary parse / send-armed gate (4 cases)
• test_status_transfer — the transfer tagged-union across all 4 states
• test_agent_mcp — tools/list count + zmodem_send discoverability

NOT built by the host suite — needs the CI product build + hardware run:

• agent_server.cpp (be_zmodem_send, status fields) and filesys_proto.cpp (ProtoEnd hook, ProtoGetLastResult) are Windows-only. They are code-reviewed for signature/type/linkage but uncompiled and unexercised here.

The two gates that remain: (1) the clang-cl+xwin build compiles the Windows halves; (2) a hardware e2e (rz → zmodem_send → poll status.transfer → checksum) proves behavior — and confirms driving the built-in sender over the existing session sidesteps the ~64 KB client-bridge ceiling.

TDD trail

RED→GREEN on the parse/gate layer (test written and observed failing — unknown method → ok:false — before call_zmodem_send existed), then the GUI-side and status hook added with the one host-testable half (surfacing) covered by test_status_transfer.

https://claude.ai/code/session_01YEkLFtTYQZuQHgQP5qYtq7