Support requesting VFS hydration; fix Android TLS cross-compilation

Correcting the previous commit: I claimed VFS placeholders could not be
downloaded. That was wrong. The desktop client exposes a socket at
$XDG_RUNTIME_DIR/Nextcloud/socket speaking newline-delimited
COMMAND:argument, and MAKE_AVAILABLE_LOCALLY:<path> does fetch the file.
Verified against client 4.0.7: a 1-byte stub became a real 2.8MB file in
2.8 seconds.

Implemented as dr-sync-nextcloud::desktop_client, deliberately optional.
Android has no desktop client, no XDG_RUNTIME_DIR socket and no
placeholders, so detect() returns None there and callers fall back to the
connector. It earns its place only because it is ~30 lines with no
dependencies: where a library already lives in a VFS folder, asking the
client to fetch beats downloading a second copy over WebDAV and leaving
the client's placeholder state inconsistent.

What this does not change: hydration is whole-file, so it suits the
original tier and never browsing. Filling a grid this way downloads the
entire library. Range extraction remains the only mechanism satisfying
FR-NC-3, and ARCH §9.0 now says so precisely.

Also fixes two real Android build failures found by cross-compiling:

  - reqwest's `rustls` feature defaults to aws-lc-rs, whose aws-lc-sys
    crate is C and fails under the NDK — exactly the pain D1 chose Rust
    to avoid. Switched to rustls-no-provider + ring, installing the
    provider in the constructor so no caller can build a client that
    panics on first use.
  - ring itself needs CC/AR per target; cargo-ndk sets only the linker.
    Added them to the container.

87 tests passing. dr-sync-nextcloud cross-compiles for aarch64-linux-android.

docs/traceability.md

@@ -9,8 +9,8 @@ Denominators are parsed from [`requirements.md`](requirements.md) at run time, n
 
 | Metric | Value |
 |---|---|
-| Source files scanned | 23 |
-| TRACES tags found | 30 |
+| Source files scanned | 25 |
+| TRACES tags found | 31 |
 | Requirements defined | 143 |
 | Requirements covered | 32 |
 | **Coverage** | **22.4%** (32/143) |
@@ -37,18 +37,18 @@ _None._
 | FR-CAT-1a | [`core/dr-types/src/lib.rs:23`](../core/dr-types/src/lib.rs#L23) |
 | FR-CAT-2 | [`tools/traceability/src/lib.rs:473`](../tools/traceability/src/lib.rs#L473) |
 | FR-CAT-5 | [`core/dr-decode/src/lib.rs:125`](../core/dr-decode/src/lib.rs#L125) |
-| FR-CAT-9 | [`core/dr-types/src/lib.rs:80`](../core/dr-types/src/lib.rs#L80) |
+| FR-CAT-9 | [`core/dr-sync-nextcloud/src/desktop_client.rs:30`](../core/dr-sync-nextcloud/src/desktop_client.rs#L30), [`core/dr-types/src/lib.rs:80`](../core/dr-types/src/lib.rs#L80) |
 | FR-CULL-1 | [`core/dr-decode/src/preview.rs:96`](../core/dr-decode/src/preview.rs#L96) |
 | FR-CULL-2 | [`core/dr-decode/src/preview.rs:123`](../core/dr-decode/src/preview.rs#L123) |
 | FR-DEV-4 | [`core/dr-gpu/src/lib.rs:119`](../core/dr-gpu/src/lib.rs#L119) |
 | FR-DSP-1 | [`ui/dr-ui/src/lib.rs:21`](../ui/dr-ui/src/lib.rs#L21) |
 | FR-EXP-9 | [`core/dr-decode/src/lib.rs:151`](../core/dr-decode/src/lib.rs#L151) |
 | FR-NC-1 | [`core/dr-sync-nextcloud/src/auth.rs:132`](../core/dr-sync-nextcloud/src/auth.rs#L132), [`core/dr-sync-nextcloud/src/auth.rs:44`](../core/dr-sync-nextcloud/src/auth.rs#L44) |
-| FR-NC-12 | [`core/dr-sync-nextcloud/src/lib.rs:30`](../core/dr-sync-nextcloud/src/lib.rs#L30), [`core/dr-sync/src/lib.rs:128`](../core/dr-sync/src/lib.rs#L128), [`core/dr-sync/src/lib.rs:34`](../core/dr-sync/src/lib.rs#L34) |
+| FR-NC-12 | [`core/dr-sync-nextcloud/src/lib.rs:32`](../core/dr-sync-nextcloud/src/lib.rs#L32), [`core/dr-sync/src/lib.rs:128`](../core/dr-sync/src/lib.rs#L128), [`core/dr-sync/src/lib.rs:34`](../core/dr-sync/src/lib.rs#L34) |
 | FR-NC-3 | [`core/dr-decode/src/preview.rs:123`](../core/dr-decode/src/preview.rs#L123), [`core/dr-sync/src/capability.rs:41`](../core/dr-sync/src/capability.rs#L41) |
 | FR-NC-4 | [`core/dr-sync-nextcloud/src/propfind.rs:100`](../core/dr-sync-nextcloud/src/propfind.rs#L100), [`core/dr-sync-nextcloud/src/propfind.rs:51`](../core/dr-sync-nextcloud/src/propfind.rs#L51), [`core/dr-sync/src/capability.rs:6`](../core/dr-sync/src/capability.rs#L6), [`core/dr-sync/src/lib.rs:128`](../core/dr-sync/src/lib.rs#L128) |
 | FR-NC-5 | [`core/dr-sync-nextcloud/src/propfind.rs:51`](../core/dr-sync-nextcloud/src/propfind.rs#L51) |
-| FR-NC-6c | [`core/dr-types/src/lib.rs:131`](../core/dr-types/src/lib.rs#L131), [`core/dr-types/src/lib.rs:80`](../core/dr-types/src/lib.rs#L80) |
+| FR-NC-6c | [`core/dr-sync-nextcloud/src/desktop_client.rs:30`](../core/dr-sync-nextcloud/src/desktop_client.rs#L30), [`core/dr-types/src/lib.rs:131`](../core/dr-types/src/lib.rs#L131), [`core/dr-types/src/lib.rs:80`](../core/dr-types/src/lib.rs#L80) |
 | FR-PLAT-AND-1 | [`core/dr-types/src/lib.rs:23`](../core/dr-types/src/lib.rs#L23) |
 | FR-RAW-1 | [`core/dr-decode/src/lib.rs:83`](../core/dr-decode/src/lib.rs#L83), [`core/dr-types/src/lib.rs:90`](../core/dr-types/src/lib.rs#L90) |
 | FR-RAW-3 | [`core/dr-decode/src/lib.rs:151`](../core/dr-decode/src/lib.rs#L151) |