Harden packaging for a public release

Add the GPLv3 text (the package declared the licence but never shipped it),
a py.typed marker, and an sdist include/exclude allowlist so the distribution
carries only src/, docs/, README, LICENSE and pyproject. reference/ is a clone
of the GPL FreeRouting Java tree kept purely as a porting reference and must
never be redistributed inside this package; tests, caches and build output are
excluded too.

Switch to the PEP 639 SPDX licence expression with license-files, widen the
classifiers, and add an -o alias for -do. Rewrite the README around the engine
matrix: what each track and pass actually does, and what this is not (no
FreeRouting density parity on dense boards, and --diagonal is a recovery and
shortening pass, not a diagonal-native search).

pyproject.toml

@@ -7,15 +7,30 @@ name = "freeroute"
 version = "0.1.0"
 description = "Native Python PCB autorouter — a Java-free port of the FreeRouting engine (Specctra DSN in, SES out)"
 readme = "README.md"
-license = { text = "GPL-3.0-or-later" }
+license = "GPL-3.0-or-later"
+license-files = ["LICENSE"]
 authors = [{ name = "Ryan Malloy", email = "ryan@supported.systems" }]
-keywords = ["pcb", "autorouter", "routing", "specctra", "dsn", "ses", "kicad", "eda"]
+keywords = [
+    "pcb",
+    "autorouter",
+    "routing",
+    "specctra",
+    "dsn",
+    "ses",
+    "kicad",
+    "eda",
+    "freerouting",
+]
 classifiers = [
-    "Development Status :: 2 - Pre-Alpha",
+    "Development Status :: 3 - Alpha",
+    "Environment :: Console",
     "Intended Audience :: Developers",
     "Intended Audience :: Manufacturing",
+    "Operating System :: OS Independent",
     "Topic :: Scientific/Engineering :: Electronic Design Automation (EDA)",
+    "Typing :: Typed",
     "Programming Language :: Python :: 3",
+    "Programming Language :: Python :: 3 :: Only",
     "Programming Language :: Python :: 3.10",
     "Programming Language :: Python :: 3.11",
     "Programming Language :: Python :: 3.12",
@@ -27,6 +42,8 @@ dependencies = []
 [project.urls]
 "Homepage" = "https://git.supported.systems/warehack.ing/freeroute"
 "Repository" = "https://git.supported.systems/warehack.ing/freeroute"
+"Issues" = "https://git.supported.systems/warehack.ing/freeroute/issues"
+# The Java engine this is a port of; freeroute ships no upstream code.
 "Upstream" = "https://github.com/freerouting/freerouting"
 
 [project.scripts]
@@ -42,6 +59,32 @@ dev = [
 [tool.hatch.build.targets.wheel]
 packages = ["src/freeroute"]
 
+[tool.hatch.build.targets.sdist]
+# Ship the package, the licence, and the docs — nothing else. `reference/` is a
+# clone of the GPL FreeRouting Java tree used only as a porting reference; it is
+# never committed and must never be redistributed inside this package.
+include = [
+    "/src",
+    "/docs",
+    "/README.md",
+    "/LICENSE",
+    "/pyproject.toml",
+]
+exclude = [
+    "/reference",
+    "/tests",
+    "/.github",
+    "/dist",
+    "/build",
+    "/CLAUDE.md",
+    "**/.env",
+    "**/__pycache__",
+    "**/*.py[cod]",
+    "**/.pytest_cache",
+    "**/.ruff_cache",
+    "**/.coverage",
+]
+
 [tool.ruff]
 target-version = "py310"
 line-length = 100