fix(headless): S1 ACDREAM_HEADLESS_CONSOLE=0 disables even on a real terminal

HeadlessConsoleOptions.Resolve tested the environment variable against
the literal "1", so ACDREAM_HEADLESS_CONSOLE=0 silently fell through to
the terminal-shaped default (on when stdin is a real console) instead of
acting as an A/B off-switch. Now: once the variable is SET AT ALL, any
value other than "0" enables and "0" disables -- the same idiom
ACDREAM_RETAIL_CLOSE_DEGRADES / ACDREAM_RETAIL_UI already use. An unset
variable still falls through to the terminal default.

Registered the flag as the sixth entry in
LaunchOptionsDocumentationTests.DefaultOnBehaviorFlags and updated the
Conventions section of docs/launch-options.md plus the flag's own row
(side-effects column corrected to describe the real precedence).

ResolvePrefersFlagThenEnvironmentThenTerminalDefault's env="0"/terminal
=true case was shown to fail against the prior `== "1"` implementation
(expected false, old code returned true) before the fix landed; the
env="yes" case also failed on the same mutation (old code required the
literal "1", so "yes" fell through to terminal=false instead of enabling).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/launch-options.md

@@ -40,12 +40,16 @@ Assume a flag has a side effect until its row says otherwise.
 - **Everything diagnostic is OFF by default.** Every probe, dump, capture,
   and measurement flag in this document is inert until its variable is
   explicitly set — an unset environment runs zero diagnostics. Exactly
-  five flags default ON, and none is a diagnostic: `ACDREAM_RETAIL_CHASE`,
+  six flags default ON, and none is a diagnostic: `ACDREAM_RETAIL_CHASE`,
   `ACDREAM_CAMERA_COLLIDE`, `ACDREAM_CAMERA_ALIGN_SLOPE`, and
   `ACDREAM_RETAIL_CLOSE_DEGRADES` are retail *behaviors* wearing an A/B
   off-switch (`=0` disables the behavior for a comparison run), while
   `ACDREAM_RETAIL_UI` is the product's only gameplay presentation and uses
-  the same explicit diagnostic opt-out. That five-flag set is frozen by
+  the same explicit diagnostic opt-out. `ACDREAM_HEADLESS_CONSOLE` is the
+  sixth: its unset default is terminal-shaped (on when stdin is a real
+  console, off when redirected — not unconditionally on like the other
+  five), but once the variable is SET AT ALL it uses the identical `=0`
+  override (any other value enables). That six-flag set is frozen by
   `LaunchOptionsDocumentationTests` — a new
   default-on flag fails the build.
 - `=1` means the code tests for exactly the string `1`. Setting `true`,
@@ -93,7 +97,7 @@ dotnet run --project src\AcDream.App\AcDream.App.csproj --no-build -c Release
 | `ACDREAM_DAT_DIR` | `=<path>` | Fallback dat-directory when no positional argument is given. App: single read at `Program.cs:58`. Cli: read independently per-subcommand (each subcommand does `args.ElementAtOrDefault(N) ?? Env.GetEnvironmentVariable("ACDREAM_DAT_DIR")`) plus once more for the default (no-subcommand) asset-inventory mode at line 152. | Two of the four `Program.cs` line numbers in the raw grep (91, 135) are **not reads** — they're the literal string `ACDREAM_DAT_DIR` inside `Log.Error` usage-text messages, not `GetEnvironmentVariable` calls. Only line 58 is a real read in `AcDream.App`. | none — hard usage error (exit 2) if unset and no positional arg | `Program.cs:58` (App); `Cli/Program.cs:24,35,47,59,71,84,113,125,137,152` (every Cli subcommand) |
 | `ACDREAM_DISPLAY_PROTOCOL` | `="auto"` / `"x11"` / `"wayland"` (case-insensitive, trimmed); any other value throws `InvalidOperationException` at startup | Linux-only: forces the GLFW 3.4 platform-init hint (X11 vs Wayland vs auto) before any window is created; ignored entirely on Windows (always `Windows` protocol) | An invalid value is fatal at startup (throws before any window exists), not a silent fallback | unset → auto-detected from `XDG_SESSION_TYPE`/`WAYLAND_DISPLAY`/`DISPLAY`, falling back to GLFW `Automatic` | `GraphicalWindowBackendSelection.Resolve` (`GraphicalWindowBackendSelection.cs:26-58`) |
 | `ACDREAM_FAR_RADIUS` | `=<int>` | Overrides preset's `FarRadius` (outer streaming/reveal window, landblocks) | Enlarging changes streaming memory budget and what's resident/rendered — CLAUDE.md: leave unset for measurement/gate runs (same family as legacy `ACDREAM_STREAM_RADIUS`) | preset's `FarRadius` (Low=5, Medium=8, High=12, Ultra=15) | `QualitySettings.WithEnvOverrides` (`QualityPreset.cs:47`) |
-| `ACDREAM_HEADLESS_CONSOLE` | `=1` enables | Turns on the headless host's interactive console (docs/plans/2026-09-07-headless-console.md): a background thread reads stdin lines, each drained on the session tick through the SAME plugin-verb/client-slash-command pipeline the graphical chat box uses, with chat/lifecycle/portal output rendered to stdout. Only takes effect for `run` with a single configured session — the launcher's multi-session mode is unaffected. | Starts a background stdin-reader thread and writes plain-text lines to the same stdout stream `HeadlessDiagnosticWriter` already uses for its JSON lines — the two interleave. Only applies to `run`; `--console` (bare flag, no value) always wins over this variable, which in turn always wins over the terminal-shaped default. | unset → on when stdin is a real console, off when redirected (`!Console.IsInputRedirected`, checked once in `Program.cs`) | `HeadlessConsoleOptions.Resolve` (`Configuration/HeadlessConsoleOptions.cs`) → `HeadlessEntryPoint.Run` → `HeadlessProcessHost`'s `consoleEnabled` |
+| `ACDREAM_HEADLESS_CONSOLE` | `=0` disables (once set at all); any other value enables; unset falls through to the terminal-shaped default | Turns on the headless host's interactive console (docs/plans/2026-09-07-headless-console.md): a background thread reads stdin lines, each drained on the session tick through the SAME plugin-verb/client-slash-command pipeline the graphical chat box uses, with chat/lifecycle/portal output rendered to stdout. Only takes effect for `run` with a single configured session — a multi-session process reports `console: single-session only` via the diagnostics stream and does not attach one. | Starts a background stdin-reader thread and writes plain-text lines to the same stdout stream `HeadlessDiagnosticWriter` already uses for its JSON lines — the two interleave. Only applies to `run`; `--console` (bare flag, no value) always wins over this variable. S1 fix (2026-09-07): the variable itself now wins outright once SET AT ALL — `=0` disables even when stdin is a real terminal, matching every other `=0`-disables flag in this table; only an UNSET variable falls through to the terminal-shaped default. | unset → on when stdin is a real console, off when redirected (`!Console.IsInputRedirected`, checked once in `Program.cs`); set → `!= "0"` | `HeadlessConsoleOptions.Resolve` (`Configuration/HeadlessConsoleOptions.cs`) → `HeadlessEntryPoint.Run` → `HeadlessProcessHost`'s `consoleEnabled` |
 | `ACDREAM_LIVE` | `=1` (exactly the literal string `"1"`) | Core switch: connect to a live ACE server instead of running offline/no-connect. | The 4 non-`RuntimeOptions.cs` line numbers in the raw grep are **all comments or log-message text**, not reads — `SessionStartComposition.cs:39` is inside the string `"live: ACDREAM_LIVE set but TEST_USER/TEST_PASS missing; skipping"`; `Program.cs:126` is inside a `--session-config` override log line; `GameWindow.cs:614,627` are doc comments. The only actual parse is `RuntimeOptions.cs:141`. Requires `ACDREAM_TEST_USER`/`ACDREAM_TEST_PASS` too (`HasLiveCredentials`) or the session silently reports `MissingCredentials` and skips. Forced to effectively-on (LiveMode=true) unconditionally by `--session-config` launches regardless of this var. | `false` | `RuntimeOptions.LiveMode` → `SessionStartComposition.cs` (log text only), `Program.cs:126` (log text only), `GameWindow.cs:614,627` (comments only), consumed for real via `RuntimeOptions.HasLiveCredentials` and `WorldSession`/`GameRuntime` session-start gating |
 | `ACDREAM_MAX_COMPLETIONS_PER_FRAME` | `=<int>` | Overrides preset's per-frame streaming-completion throughput cap | Directly changes the streaming admission budget measured by perf/completion gates — do not vary during a measurement run | preset's value (Low=2, Medium=3, High=4, Ultra=6) | `QualitySettings.WithEnvOverrides` (`QualityPreset.cs:59`) |
 | `ACDREAM_MSAA_SAMPLES` | `=<int>` (0/2/4/8) | Overrides preset's MSAA sample count | Changes GPU multisample anti-aliasing (visual + GPU-cost change) | preset's `MsaaSamples` (Low=0, Medium=2, High/Ultra=4) | `QualitySettings.WithEnvOverrides` (`QualityPreset.cs:48`) |