refactor(gamestream/host): source gamepad BTN_* from punktfunk_core + pin the wire bits (G13/G15)

`gamestream/gamepad.rs` hand-declared its own copy of the GameStream
buttonFlags/buttonFlags2 layout, which had drifted from the single source
of truth in `punktfunk_core::input::gamepad`: the click bits were named
`BTN_LS_CLK`/`BTN_RS_CLK` (vs core's `…_CLICK`). The two layouts are
bit-identical — GameStream/Limelight and the punktfunk/1 native wire are
one contract — so define the gamestream names as `pub const` aliases of
the core constants. Values now come solely from core (can't drift);
kept as `pub const` (not a `pub use` re-export) because on Windows the
only consumer — the Linux uinput button map — is cfg'd out, where an
unused re-export lints as an error but an unused pub const does not.

Rename the two injector call-sites (`inject/linux/gamepad.rs`) to the
canonical `BTN_LS_CLICK`/`BTN_RS_CLICK`.

G15 host half: replace the 3-bit gamestream-vs-core spot-check with an
exhaustive golden-value test (`gamepad_wire_bits_are_pinned`) that freezes
every button bit + axis id to its exact wire value, so renumbering a bit
in core — which would silently break every shipped client — fails a test
first. The host counterpart to the client-side C-ABI cross-checks.

Verified on Linux .21: clippy -D warnings clean, pin test + gamepad
suite green. (Windows verified together with the rest of Phase 3.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/src/inject/linux/gamepad.rs

@@ -89,8 +89,8 @@ const BUTTON_MAP: [(u32, u16); 15] = [
     (gamepad::BTN_BACK, BTN_SELECT),
     (gamepad::BTN_START, BTN_START),
     (gamepad::BTN_GUIDE, BTN_MODE),
-    (gamepad::BTN_LS_CLK, BTN_THUMBL),
-    (gamepad::BTN_RS_CLK, BTN_THUMBR),
+    (gamepad::BTN_LS_CLICK, BTN_THUMBL),
+    (gamepad::BTN_RS_CLICK, BTN_THUMBR),
     (gamepad::BTN_PADDLE1, BTN_TRIGGER_HAPPY5),
     (gamepad::BTN_PADDLE2, BTN_TRIGGER_HAPPY6),
     (gamepad::BTN_PADDLE3, BTN_TRIGGER_HAPPY7),