merge fix-ups: the four review findings + a build portability gate the merge exposed

Applied on top of the glass-cockpit-refit merge, all from the pre-merge review:

1. README CONTROLS TABLE REWRITTEN for the new default board.  It still taught
   "W/S throttle, A/D turn" while the defaults moved flight to the numpad and
   handed the letter rows to the MFD banks -- a fresh install would read
   controls that do not work.  The new table teaches the numpad flight block,
   the panel-on-your-keys groups, F7 for control mode (was M), backtick for the
   camera (was V), the coolant valves on 1-3/QWE with the detent warning kept,
   and PgUp/PgDn volume.  UPGRADERS ARE TOLD EXPLICITLY that their preserved
   bindings.txt keeps their old keys, and that deleting it opts into the board.

2. BACKTICK VIEW-TOGGLE GUARDED.  The bindings-row-wins rule (KeyHasBinding)
   covered V and J/K/L but not backtick itself, which polled unconditionally --
   and BACKTICK is a nameable, unbound key, so binding it to a button
   double-dispatched (button + camera).  Both halves of the view poll are now
   guarded.

3. ENVIRON.INI REFUSES ONE-SHOTS.  The loader applied ANY key=value line, and
   BT_JOYCONFIG in the file would defeat the #66 one-shot in a sneaky way: the
   wizard DELETES the process var after running, so in every relaunched child
   there is no real env var left to win over the file -- capture wizard at
   every mission start.  BT_JOYCONFIG is now skipped with the reason in the
   template header, which also warns that test/debug hooks (BT_MP_FORCE_DMG
   and friends) do not belong in a file that silently persists forever.

4. VOLUME DOCUMENTED EVERYWHERE IT WAS MISSING.  CONTROLS.md gains the
   PgUp/PgDn row (the old "- / = (see below)" cell pointed at a note that never
   mentioned volume) and the note that the keys moved so they no longer share a
   key with anything in any layout; the bindings template header reserves
   PgUp/PgDn informally.

5. L4KEYLIGHT BUILD GATE (found by building the merge on this machine).  The
   C++/WinRT Dynamic Lighting TU does not compile against SDK 10.0.19041 --
   its bundled cppwinrt fails inside winrt/base itself (C2039 'wait_for').
   CMake now detects an SDK older than 10.0.22000 and builds a dormant stub
   with the same five-function scalar interface (logs once at Start, identical
   behaviour otherwise) -- extending the feature's own runtime philosophy
   ("no Dynamic Lighting -> log once, stay dormant") to build time.  Dynamic
   Lighting is a Windows 11 22H2 feature, so nothing real is lost on an older
   build machine, and machines with a newer SDK build the real mirror
   unchanged.

VERIFIED on the merged tree (4.11.572):
  * build clean -- 0 compile errors; the 40 /FORCE-tolerated unresolved
    externals are byte-identical to every pre-merge build (20
    CreateStreamedSubsystem + 20 DefaultData; the earlier claim that all 40
    were CSS was shorthand -- corrected here for the record).
  * solo boot: environ.ini template written + loaded, mode resolver announces
    the surround, historical bands L276/R276/T223/B336 confirmed, keylight
    stub logs its dormant line, BT_SHOT capture shows the under-glass button
    treatment and the corrected hat labels, 0 faults.
  * PgUp/PgDn volume PROVEN LIVE via injected keys: 5%->10%->15%->10%, saved
    to volume.cfg.  (Side catch: content\volume.cfg had been sitting at 0.00
    from an old test -- anyone using this tree had a silent game; reset.)
  * full relay cycle on the merged exe: 2 pods staged, launch pair, mission,
    StopMission, round RESET -- all clean.
  * all three console suites still pass; checkctx CLEAN.

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

docs/CONTROLS.md

@@ -23,7 +23,7 @@ Flight lives on the number pad so the board stays free.
 | NumPad 0, Space | main trigger | | NumPad 1 / 3 / . | pinky / middle / upper |
 | Shift / Ctrl | throttle up / down (the lever sticks) | | Alt | reverse thrust |
 | ← ↓ → | look left / behind / right | | ↑ | torso centre |
-| ` | cockpit / chase camera | | − / = *(see below)* | — |
+| ` | cockpit / chase camera | | PgUp / PgDn | volume up / down (saved) |
 
 **You spawn in BASIC control mode.** In BASIC the stick steers and the pedals do nothing.
 Press **F7** once for MID, where the pedals steer and the stick twists the torso — the full
@@ -66,7 +66,8 @@ ammo bay before it detonates).
 Binding a key removes it from the authentic 1995 typed-hotkey channel so it cannot
 double-dispatch. This board binds nearly everything, so those hotkeys are given up by design:
 `5` = MFD1 Quad page, `z` = MFD3 Eng1, `t/y/u/i/o` = pilot select, `+`/`-` = target zoom.
-Unbind a key in `bindings.txt` to get its 1995 meaning back. Same rule for the built-ins:
+Unbind a key in `bindings.txt` to get its 1995 meaning back. (The old `-`/`=` VOLUME keys
+moved to **PgUp/PgDn** -- they no longer share a key with anything, in any layout.) Same rule for the built-ins:
 **`** is the view toggle (V is a board button now), and J/K/L cycle the MFD preset pages only
 while unbound.