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).

tests/ses/test_indent.py

@@ -0,0 +1,63 @@
+"""Tests for the SES indent writer and identifier quoting."""
+
+from __future__ import annotations
+
+from freeroute.ses.indent import Identifier, IndentWriter
+
+
+def test_nested_scopes_indent_by_two_spaces():
+    out = IndentWriter()
+    out.start_scope(new_line=False)
+    out.write("session x")
+    out.start_scope()
+    out.write("routes")
+    out.end_scope()
+    out.end_scope()
+    assert out.getvalue() == "(session x\n  (routes\n  )\n)"
+
+
+def test_new_line_uses_current_indent():
+    out = IndentWriter()
+    out.start_scope(new_line=False)
+    out.write("a")
+    out.new_line()
+    out.write("(b)")
+    out.end_scope()
+    assert out.getvalue() == "(a\n  (b)\n)"
+
+
+def test_plain_name_not_quoted():
+    ident = Identifier()
+    assert ident.format("Top") == "Top"
+    assert ident.format("U1") == "U1"
+    assert ident.format("F.Cu") == "F.Cu"  # '.' is not reserved
+
+
+def test_reserved_chars_force_quotes():
+    ident = Identifier()
+    assert ident.format("NET_A") == '"NET_A"'  # underscore reserved
+    assert ident.format("a-b") == '"a-b"'  # hyphen reserved
+    assert ident.format("with space") == '"with space"'
+    assert ident.format("Via[0-1]_600:300_um") == '"Via[0-1]_600:300_um"'
+
+
+def test_leading_digit_forces_quotes():
+    ident = Identifier()
+    assert ident.format("2layer") == '"2layer"'
+    assert ident.format("-3x") == '"-3x"'
+
+
+def test_non_ascii_forces_quotes():
+    ident = Identifier()
+    assert ident.format("Паяльная") == '"Паяльная"'
+
+
+def test_surrounding_quotes_stripped_before_reprocessing():
+    ident = Identifier()
+    # already-quoted plain name comes back unquoted
+    assert ident.format('"Top"') == "Top"
+
+
+def test_embedded_quote_removed():
+    ident = Identifier()
+    assert ident.format('a"b') == "ab"

tests/ses/test_writer.py

@@ -0,0 +1,139 @@
+"""End-to-end tests for the SES writer.
+
+Where possible these re-parse the emitted SES with the project's own tokenizer /
+S-expression parser (:mod:`freeroute.dsn.sexp`) and assert structural
+invariants, mirroring what FreeRouting's ``SesReader`` and ``kicad-cli pcb
+import specctra-ses`` walk: ``session > routes > network_out > net >
+wire(path)/via``.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+from freeroute.dsn import parse_dsn
+from freeroute.dsn.sexp import SExp, parse
+from freeroute.ses import RoutedVia, RoutedWire, RoutingResult, write_ses
+
+FIXTURES = Path(__file__).parent.parent / "dsn" / "fixtures"
+
+
+def load_board(name: str):
+    return parse_dsn((FIXTURES / name).read_text())
+
+
+@pytest.fixture
+def board():
+    return load_board("smd_demo.dsn")
+
+
+def child(node: SExp, head: str) -> SExp:
+    found = node.child(head)
+    assert found is not None, f"missing ({head} ...) scope"
+    return found
+
+
+# --- no-op session -----------------------------------------------------------
+
+
+def test_noop_session_is_parseable_and_well_formed(board):
+    text = write_ses(board)
+    top = parse(text)  # must be a single balanced S-expression
+    assert top.head == "session"
+    # session name argument: design name with .dsn -> .ses
+    assert top.values()[0].text == "smd-demo.ses"
+
+
+def test_noop_session_has_required_scopes(board):
+    top = parse(write_ses(board))
+    assert child(top, "base_design").values()[0].text == "smd-demo.dsn"
+    assert top.child("placement") is not None
+    assert top.child("was_is") is not None
+    routes = child(top, "routes")
+    # routes must carry a resolution (KiCad scales session coords by it)
+    assert child(routes, "resolution").values()[0].text == "um"
+    assert routes.child("network_out") is not None
+
+
+def test_noop_network_out_is_empty(board):
+    routes = child(parse(write_ses(board)), "routes")
+    network = child(routes, "network_out")
+    assert network.children("net") == []
+
+
+def test_placement_echoes_components(board):
+    placement = child(parse(write_ses(board)), "placement")
+    comps = placement.children("component")
+    names = [c.values()[0].text for c in comps]
+    assert names == ["MiniQFN-6", "0603"]
+    # U1 place: coords rounded to int, side, rotation
+    u1 = comps[0].child("place")
+    vals = [v.text for v in u1.values()]
+    assert vals == ["U1", "75000", "-45000", "front", "90"]
+
+
+def test_library_out_echoes_via_padstack(board):
+    routes = child(parse(write_ses(board)), "routes")
+    lib = child(routes, "library_out")
+    pads = lib.children("padstack")
+    assert [p.values()[0].text for p in pads] == ["Via[0-1]_600:300_um"]
+
+
+# --- routed geometry ---------------------------------------------------------
+
+
+@pytest.fixture
+def routed():
+    result = RoutingResult()
+    result.add_wire("NET_A", RoutedWire(layer="Top", width=200, coords=[0, 0, 1000, -500]))
+    result.add_via("NET_A", RoutedVia(padstack="Via[0-1]_600:300_um", x=1000, y=-500))
+    result.add_wire("NET_B", RoutedWire(layer="Bottom", width=250, coords=[10, 10, 20, 20]))
+    return result
+
+
+def test_routed_network_out_has_nets(board, routed):
+    routes = child(parse(write_ses(board, routed)), "routes")
+    network = child(routes, "network_out")
+    nets = network.children("net")
+    assert [n.values()[0].text for n in nets] == ["NET_A", "NET_B"]
+
+
+def test_routed_wire_path_and_via(board, routed):
+    routes = child(parse(write_ses(board, routed)), "routes")
+    net_a = child(routes, "network_out").children("net")[0]
+
+    wire = child(net_a, "wire")
+    path = child(wire, "path")
+    pvals = [v.text for v in path.values()]
+    # (path <layer> <width> x1 y1 x2 y2)
+    assert pvals == ["Top", "200", "0", "0", "1000", "-500"]
+
+    via = child(net_a, "via")
+    vvals = [v.text for v in via.values()]
+    assert vvals == ["Via[0-1]_600:300_um", "1000", "-500"]
+
+
+def test_coordinates_are_integers(board):
+    result = RoutingResult()
+    result.add_wire("NET_A", RoutedWire(layer="Top", width=199.6, coords=[0.4, -0.4, 2.5, 3.5]))
+    routes = child(parse(write_ses(board, result)), "routes")
+    path = child(child(child(routes, "network_out").children("net")[0], "wire"), "path")
+    pvals = [v.text for v in path.values()]
+    # width 199.6 -> 200; coords 0.4->0, -0.4->0, 2.5->3, 3.5->4 (round half up)
+    assert pvals == ["Top", "200", "0", "0", "3", "4"]
+    assert all("." not in v for v in pvals)
+
+
+def test_empty_result_matches_noop(board):
+    assert write_ses(board, RoutingResult()) == write_ses(board)
+
+
+def test_round_trip_from_empty_board_fixture():
+    board = load_board("empty_board.dsn")
+    top = parse(write_ses(board))
+    assert top.head == "session"
+    assert top.values()[0].text == "freeroute-empty.ses"
+    # empty board has no placements -> placement scope has no components
+    assert child(top, "placement").children("component") == []