fix(ltx2): cast positions double/float to avoid MSVC C4244 (#968)

MSVC refuses the implicit narrowing in the two `StreamState` <-> `Ltx2LatentState`
position copies under `/WX`, so the native Windows build cannot compile
`ltx2_video.cpp` at all. The range-assign is replaced by an explicit
`static_cast` loop in each direction.

The cast is not a claim that precision does not matter here; it is a claim that
none is lost, and the file already carries the argument. `ltx2_video.cpp:193-196`
records that the positions round trip exactly because every value in the `double`
buffer was widened from a `float` in the first place -- the `double` exists only
because the DiT surface takes one. Both writers agree: the video axis is
`static_cast<double>(float_expr)` at `:2463-2466`, and the audio axis comes from
`Ltx2AudioPatchTimings`, which returns `std::vector<float>`, at `:2535`. So
`float -> double -> float` reproduces the bits, and the explicit cast performs
exactly the conversion the implicit assign already performed.

This closes the last MSVC compile blocker in the #503 class. The lane still fails
afterwards, later and for a different reason -- `test_openai_api_server.exe` exits
`0xC0000409` with no doctest summary, a runtime defect the build never reached
before. That is tracked separately and is not this change.

A second commit refreshes the READER ANCHORS comment. The casts add nine net
lines above `kKnownLoadExtras`, shifting every recorded anchor by nine, and
`test_ltx2_video` derives and compares that list on every run so the drift is
caught rather than silently rotting. The replacement list was derived against
main at `0f8580e26` with this branch merged, because main moved this file twice
since the branch base (`332aed738`, `3ce1cf7c7`).

Fixes #968

FOLLOWING_AGENTS_PROTOCOL

Following-Agents-Protocol: true
AI-Assisted: true
Assisted-by: ClaudeCode:claude-opus-5 [ClaudeCode]
Signed-off-by: harleywilsoneng <harleywilsoneng@users.noreply.github.com>

Author: harleywilsoneng

src/vllm/multimodal/ltx2_video.cpp

@@ -203,7 +203,13 @@ Ltx2LatentState ToLatentState(const StreamState& s, int64_t pos_dims) {
   out.latent = s.latent;
   out.clean = s.clean;
   out.mask = s.mask;
-  out.positions.assign(s.positions.begin(), s.positions.end());
+  // Explicit cast: StreamState stores positions as double for the DiT surface,
+  // Ltx2LatentState keeps float32. Range-assign would narrow implicitly and
+  // trip MSVC C4244 under /WX (mudler/vllm.cpp#968).
+  out.positions.resize(s.positions.size());
+  for (size_t i = 0; i < s.positions.size(); ++i) {
+    out.positions[i] = static_cast<float>(s.positions[i]);
+  }
   out.keyframes_mask = s.keyframes_mask;
   return out;
 }
@@ -214,7 +220,10 @@ void FromLatentState(const Ltx2LatentState& in, StreamState* s) {
   s->latent = in.latent;
   s->clean = in.clean;
   s->mask = in.mask;
-  s->positions.assign(in.positions.begin(), in.positions.end());
+  s->positions.resize(in.positions.size());
+  for (size_t i = 0; i < in.positions.size(); ++i) {
+    s->positions[i] = static_cast<double>(in.positions[i]);
+  }
   s->keyframes_mask = in.keyframes_mask;
 }
 
@@ -364,7 +373,7 @@ constexpr char kLtx2DurationHeadPathExtra[] = "duration_head_path";
 // they are no longer trusted: the list below is derived from this file on every
 // run and compared, and the failure prints the replacement to paste in.
 // READER ANCHORS (derived and gated by test_ltx2_video):
-// 782 792 793 855 951 967 969 1060 1085 1190 1231
+// 791 801 802 864 960 976 978 1069 1094 1199 1240
 const char* const kKnownLoadExtras[] = {
     kLtx2AudioPromptEmbedsExtra, kLtx2PipelineKindExtra,   kLtx2ModelVersionExtra,
     kLtx2AllowUnportedExtra,     kLtx2MaxPhaseExtra,       kLtx2DitConfigPathExtra,