feat: make crosswalk and stop-line offsets solvable

Step 1 of the control-marking task, and deliberately server-only: no handle is
drawn yet. This project already shipped a range handle for `profile.interval`,
which compileGeometry ignores, so the control dragged and changed nothing. The
consumer comes first now.

Two kinds join the taxonomy — `junction-crosswalk-inset` and
`junction-stop-line-offset`, both on the existing `junction-approach` anchor. The
solver writes them onto the approach entry, `applyDirectJunctionPlans` carries
them onto the compiled approach, and `compileControlMarkings` reads them in place
of the module constants it used for every junction. They move markings without
reshaping the junction, so unlike width and cutback they deliberately do not
trigger a boundary recompute.

`applyJunctionConstraint` becomes an explicit switch. Its trailing `else` had
meant every kind that was not approach-width fell through to the cutback
validator, so a new kind would have been silently validated and written as a
cutback. The same non-exhaustive shape in the test fixture's `valueFor` is fixed
the same way, and now throws for an unnamed kind rather than answering with a
corner radius.

design.md's taxonomy is updated with it — a test asserts the two cannot drift,
which is what caught the omission.

Measured on a 41-road workspace with 8 crossings: both constraints change their
marking geometry, neither drags the other, and out-of-range blocks instead of
clamping. That measurement is not in the suite: the synthetic junction resolves
`junction_inset_m` to 0 because its crossing never binds to a plan, and the
committed OSM fixture has no crossings at all. The tests assert the wiring the
handles will depend on — values reaching the approach entry, distinct branches,
blocking diagnostics — and the gap is recorded in the test itself.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.trellis/scripts/common/git_context.py

@@ -0,0 +1,106 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+"""
+Git and Session Context utilities.
+
+Entry shim — delegates to session_context and packages_context.
+
+Provides:
+    output_json - Output context in JSON format
+    output_text - Output context in text format
+"""
+
+from __future__ import annotations
+
+import json
+
+from .git import run_git
+from .session_context import (
+    get_context_json,
+    get_context_text,
+    get_context_record_json,
+    get_context_text_record,
+    output_json,
+    output_text,
+)
+from .packages_context import (
+    get_context_packages_text,
+    get_context_packages_json,
+)
+from .trellis_config import read_trellis_config
+from .workflow_phase import (
+    filter_platform,
+    get_phase_index,
+    get_step,
+    resolve_effective_platform,
+)
+
+# Backward-compatible alias — external modules import this name
+_run_git_command = run_git
+
+
+# =============================================================================
+# Main Entry
+# =============================================================================
+
+def main() -> None:
+    """CLI entry point."""
+    import argparse
+
+    parser = argparse.ArgumentParser(description="Get Session Context for AI Agent")
+    parser.add_argument(
+        "--json",
+        "-j",
+        action="store_true",
+        help="Output in JSON format (works with any --mode)",
+    )
+    parser.add_argument(
+        "--mode",
+        "-m",
+        choices=["default", "record", "packages", "phase"],
+        default="default",
+        help="Output mode: default (full context), record (for record-session), packages (package info only), phase (workflow step extraction)",
+    )
+    parser.add_argument(
+        "--step",
+        help="Step id for --mode phase, e.g. 1.1, 2.2. Omit to get the Phase Index.",
+    )
+    parser.add_argument(
+        "--platform",
+        help="Platform name for --mode phase, e.g. cursor, claude-code. Filters platform-tagged blocks.",
+    )
+
+    args = parser.parse_args()
+
+    if args.mode == "record":
+        if args.json:
+            print(json.dumps(get_context_record_json(), indent=2, ensure_ascii=False))
+        else:
+            print(get_context_text_record())
+    elif args.mode == "packages":
+        if args.json:
+            print(json.dumps(get_context_packages_json(), indent=2, ensure_ascii=False))
+        else:
+            print(get_context_packages_text())
+    elif args.mode == "phase":
+        content = get_step(args.step) if args.step else get_phase_index()
+        if not content.strip():
+            if args.step:
+                parser.exit(2, f"Step not found: {args.step}\n")
+            else:
+                parser.exit(2, "Phase Index section not found in workflow.md\n")
+        if args.platform:
+            effective = resolve_effective_platform(
+                args.platform, read_trellis_config()
+            )
+            content = filter_platform(content, effective)
+        print(content, end="")
+    else:
+        if args.json:
+            output_json()
+        else:
+            output_text()
+
+
+if __name__ == "__main__":
+    main()