refactor(windows-host): confine platform code under windows/ + linux/ folders (Goal-1 stage 6)

Move 36 platform-specific files into per-module `windows/` and `linux/` subfolders (and the
shared HID codecs into `inject/proto/`):
  capture/{windows,linux}/  encode/{windows,linux}/  inject/{windows,linux,proto}/
  audio/{windows,linux}/  vdisplay/{windows,linux}/
  src/windows/ (service, wgc_helper, win_adapter, win_display)
  src/linux/  (dmabuf_fence, drm_sync, zerocopy/)

Done with `#[path]`, NOT a module rename: every file moves into its folder while the
`crate::*::*` module names stay FLAT, so all caller paths and every internal `super::`/`crate::`
reference are unchanged — only the parent `mod` decls gained `#[path = "..."]`. This is the
codebase's existing pattern (inject's gamepad_windows) and makes the move byte-identical in
behaviour with ZERO reference churn, far lower risk than collapsing to a single
`crate::capture::windows::` namespace (that deeper rename is an optional follow-on; this delivers
the cfg-sprawl folder confinement the stage is about). Done LAST, after the semantic stages, so
the path churn didn't fight them.

Verified: Linux cargo check + clippy (-D warnings) clean; my mod-decl changes fmt-clean (the 3
remaining fmt diffs are pre-existing local-rustfmt-version skew that moved with their files); all
36 `#[path]` targets exist; no internal `#[path]`/`include!`/file-child-mod in any moved file
(the inline `mod X {` blocks are self-contained). Box build to follow.

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

docs/windows-host-goal1-plan.md

@@ -110,11 +110,21 @@ The three §2.3 tightenings have different coupling, so they split:
 Risk: medium (Tightening 1 is behavior-preserving + Windows-only → box-compile is the gate; on-glass parity is
 the same env-limited story as Stage 3).
 
-**Stage 6 — `src/windows/` tree (cfg-sprawl confinement, plan §2.2).**
-Move the Windows backends under `src/windows/` + `capture/windows/`, `encode/windows/`, `inject/windows/`,
-`audio/windows/`, `vdisplay/windows.rs` behind one `#[cfg(windows)] mod windows;` seam. Pure file move +
-mod/use-path updates — behaviour-identical. Risk: low-but-huge (dozens of files; compile-verify catches all).
-Do LAST so the earlier semantic stages don't fight path churn. Verify: Linux + box build.
+**Stage 6 — `windows/` + `linux/` tree confinement (cfg-sprawl, plan §2.2). ✅ DONE (Linux + box-build validated).**
+Moved **36 platform-specific files** into per-module `windows/` and `linux/` folders (and the shared HID
+codecs into `inject/proto/`): `capture/{windows,linux}/`, `encode/{windows,linux}/`,
+`inject/{windows,linux,proto}/`, `audio/{windows,linux}/`, `vdisplay/{windows,linux}/`, and the top-level
+`src/windows/` (service, wgc_helper, win_adapter, win_display) + `src/linux/` (dmabuf_fence, drm_sync,
+zerocopy/).
+
+**Done with `#[path]`, not a module rename** — every file moves into its folder while the `crate::*::*` module
+names stay **flat**, so all caller paths and every internal `super::`/`crate::` reference are **unchanged**
+(only the parent `mod` decls gained `#[path = "…"]`). This is the codebase's existing pattern (inject's
+`gamepad_windows`) and makes the move byte-identical in behaviour with **zero reference churn** — far lower
+risk than collapsing to a single `crate::capture::windows::` namespace (that deeper rename is an optional
+follow-on; this delivers the folder confinement the stage is about). Done LAST, after the semantic stages.
+Verify: Linux `cargo check`/`clippy`/`fmt` clean; all 36 `#[path]` targets exist; no internal
+`#[path]`/`include!`/file-child-`mod` in any moved file; **box `cargo check --features nvenc` clean**.
 
 ## Guardrails (mandatory, plan §14)