tools/jtag/mpsse_jtag.py: take the libftdi path from the environment, so B2 reads on Linux #7927

Problem. tools/jtag/mpsse_jtag.py loads libftdi from a fixed Homebrew path:

LIB = "/opt/homebrew/lib/libftdi1.2.dylib"

On Linux the file is not there, so read_verdict.py (stage B2 of t27c silicon) fails with OSError: /opt/homebrew/lib/libftdi1.2.dylib: cannot open shared object file. This stopped die C of R3-3 (#7669) in the Linux lab; the run passed only after a symlink at that path inside the container (#7906, "Caveats").

Change (5 lines, one file).

import ctypes, ctypes.util, os, sys, time

# libftdi1: $T27_LIBFTDI if set; else Homebrew on macOS; else the loader search (Linux).
LIB = os.environ.get("T27_LIBFTDI") or (
    "/opt/homebrew/lib/libftdi1.2.dylib" if sys.platform == "darwin"
    else ctypes.util.find_library("ftdi1") or "libftdi1.so.2")

macOS keeps today's path, so nothing changes on the existing benches.

Why no .t27 first. The file is hand-written Python, and the "Only t27" gate (specs/policy/own_language.t27) refuses it unless tools/policy/foreign-exceptions.txt lists it. There is no Python back end, so a spec cannot generate this file; the port specs/port/tools/jtag/mpsse_jtag.t27 is a stub. So the route is an owner-approved exception for this one file, then the 5-line change (well inside the 40-line budget).

Done when.

1. tools/jtag/mpsse_jtag.py is listed in foreign-exceptions.txt, approved by the owner (label owner-approved-foreign).
2. The change above is on master; python3 -c "import sys; sys.path.insert(0,'tools/jtag'); import mpsse_jtag" loads on Linux without a symlink, and t27c silicon B2 passes there.

Refs #7669, #7906.

User Scenarios

• Given a Linux machine without a symlink at /opt/homebrew/lib/libftdi1.2.dylib, when t27c silicon runs stage B2 (read_verdict.py), then mpsse_jtag.py loads libftdi via ctypes.util.find_library("ftdi1") and the stage passes.
• Given T27_LIBFTDI=/usr/lib/x86_64-linux-gnu/libftdi1.so.2 in the environment on Linux, when Python imports mpsse_jtag, then the module uses that exact path and loads successfully.

Requirements

• FR-001: tools/jtag/mpsse_jtag.py MUST read T27_LIBFTDI from the environment and use it as the library path when set.
• FR-002: On macOS (sys.platform == "darwin"), the default path MUST remain /opt/homebrew/lib/libftdi1.2.dylib.
• FR-003: On non-macOS platforms, the default MUST be ctypes.util.find_library("ftdi1") or "libftdi1.so.2" as fallback.
• FR-004: tools/jtag/mpsse_jtag.py MUST be listed in tools/policy/foreign-exceptions.txt with owner approval (owner-approved-foreign label).

Success Criteria

• python3 -c "import sys; sys.path.insert(0,'tools/jtag'); import mpsse_jtag" runs on Linux without a symlink and exits 0.
• grep -q "tools/jtag/mpsse_jtag.py" tools/policy/foreign-exceptions.txt exits 0.
• t27c silicon stage B2 passes on Linux (exit 0).

Boundary

• tools/jtag/mpsse_jtag.py
• tools/policy/foreign-exceptions.txt