wip: Campaign LA LA1 fix round — INCOMPLETE, stopped mid-task

Agent was stopped for token budget partway through the LA1 review fix
round. Landed here: F1 best-effort SessionStatusWriter, F2 App reader
tolerance (paths/mode), F5 argument-parsing hardening, plus new tests.
NOT DONE: F4 shared-fixture production shape (was the next step), F3
reconnect disconnected edge + recorded limitation, F6 exited
idempotency/reasons, F7 structural redaction test, F8 platform-guard
test + comment fix, optional RuntimeOptions PrintMembers redaction.

Build/test state UNVERIFIED at this commit. Next session: finish the
remaining findings, run the suites, then narrow re-review.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/AcDream.App.Tests/Configuration/SessionConfigArgumentParsingTests.cs

@@ -0,0 +1,97 @@
+using AcDream.App.Configuration;
+
+namespace AcDream.App.Tests.Configuration;
+
+/// <summary>
+/// Campaign LA slice LA1 review fix (F5): pins
+/// <see cref="SessionConfigArgumentParsing"/>'s trailing-flag edge case —
+/// <c>--session-config</c> present as the LAST argument with nothing after
+/// it must be distinguishable from the flag being entirely absent, so
+/// <c>Program.cs</c> can turn it into a hard error instead of a silent
+/// fall-through to the env-var/positional dat-dir path.
+/// </summary>
+public sealed class SessionConfigArgumentParsingTests
+{
+    private const string Flag = "--session-config";
+
+    [Fact]
+    public void FlagWithAFollowingValueReturnsThatValueAndIsPresent()
+    {
+        string? value = SessionConfigArgumentParsing.ExtractFlagValue(
+            ["D:\\dats", Flag, "session.json"],
+            Flag,
+            out bool present);
+
+        Assert.True(present);
+        Assert.Equal("session.json", value);
+    }
+
+    [Fact]
+    public void FlagAbsentReturnsNullAndIsNotPresent()
+    {
+        string? value = SessionConfigArgumentParsing.ExtractFlagValue(
+            ["D:\\dats"],
+            Flag,
+            out bool present);
+
+        Assert.False(present);
+        Assert.Null(value);
+    }
+
+    [Fact]
+    public void TrailingFlagWithNoValueIsPresentWithANullValue()
+    {
+        string? value = SessionConfigArgumentParsing.ExtractFlagValue(
+            ["D:\\dats", Flag],
+            Flag,
+            out bool present);
+
+        // This is the case Program.cs must turn into exit code 2 — present
+        // but no value is categorically different from "not present at
+        // all", even though both currently yield a null return value.
+        Assert.True(present);
+        Assert.Null(value);
+    }
+
+    [Fact]
+    public void FlagAloneAsTheOnlyArgumentIsPresentWithANullValue()
+    {
+        string? value = SessionConfigArgumentParsing.ExtractFlagValue(
+            [Flag],
+            Flag,
+            out bool present);
+
+        Assert.True(present);
+        Assert.Null(value);
+    }
+
+    [Fact]
+    public void WithoutFlagAndValueDropsTheFlagAndItsValue()
+    {
+        string[] positional = SessionConfigArgumentParsing.WithoutFlagAndValue(
+            ["D:\\dats", Flag, "session.json", "extra"],
+            Flag);
+
+        Assert.Equal(["D:\\dats", "extra"], positional);
+    }
+
+    [Fact]
+    public void WithoutFlagAndValueTrailingFlagDropsOnlyTheFlagItself()
+    {
+        string[] positional = SessionConfigArgumentParsing.WithoutFlagAndValue(
+            ["D:\\dats", Flag],
+            Flag);
+
+        Assert.Equal(["D:\\dats"], positional);
+    }
+
+    [Fact]
+    public void WithoutFlagAndValueLeavesArgumentsUnchangedWhenFlagIsAbsent()
+    {
+        string[] positional = SessionConfigArgumentParsing.WithoutFlagAndValue(
+            ["D:\\dats"],
+            Flag);
+
+        Assert.Equal(["D:\\dats"], positional);
+    }
+}