Implement Specctra SES session-file writer

Ports the write path of io/specctra/SesWriter.java to emit a valid
session from a parsed DsnBoard plus a RoutingResult. Each _write_*
function mirrors a write* method upstream:

- session scope with base_design, placement (resolution + components
  echoed from the DSN), an empty was_is, and routes
- routes carries resolution, a reduced parser scope, library_out with
  the via padstacks, and network_out
- network_out emits (net (wire (path layer width x1 y1 ...)) (via
  padstack x y)) for each routed net; integer coordinates via round-
  half-up to match Java Math.round

write_ses(board) with no result produces a valid no-op session,
proving the DSN-in / SES-out round trip. Only routes > network_out is
required by SesReader and kicad-cli; the rest is echoed for a
well-formed file.

18 tests: indent/quoting rules, no-op session structure, placement and
library_out echo, routed wire/via serialization, integer rounding, and
round-trips re-parsed through the project's own DSN tokenizer/sexp
(64 tests total across the suite, all green; ruff clean).

src/freeroute/ses/__init__.py

@@ -0,0 +1,31 @@
+"""Specctra SES (session) writing for freeroute.
+
+Public API::
+
+    from freeroute.dsn import parse_dsn
+    from freeroute.ses import write_ses, RoutingResult, RoutedWire, RoutedVia
+
+    board = parse_dsn(dsn_text)
+    ses_text = write_ses(board)                 # valid no-op session
+    # ... or with routed geometry:
+    result = RoutingResult()
+    result.add_wire("NET_A", RoutedWire("Top", 200, [0, 0, 1000, 0]))
+    ses_text = write_ses(board, result)
+"""
+
+from __future__ import annotations
+
+from .indent import Identifier, IndentWriter
+from .model import RoutedNet, RoutedVia, RoutedWire, RoutingResult
+from .writer import write_ses, write_ses_file
+
+__all__ = [
+    "write_ses",
+    "write_ses_file",
+    "RoutingResult",
+    "RoutedNet",
+    "RoutedWire",
+    "RoutedVia",
+    "Identifier",
+    "IndentWriter",
+]