feat: PuLID-Flux identity-injection support

[RapidMark]

This PR adds support for PuLID-Flux
identity preservation to the Flux denoise loop. Given a single source
portrait, generated images preserve the source person's face across
arbitrary scenes and prompts.

What's included

• src/pulid.hpp — PuLIDPerceiverAttentionCA, the cross-attention
  module mirroring the PyTorch reference at
  ToTheBeginning/PuLID/.../encoders_transformer.py.
  Pure-ggml graph; runs on CPU / CUDA / Vulkan / Metal without
  backend-specific code.
• src/flux.hpp — adds 20 pulid_ca.<i> child blocks to Flux
  (constructed conditionally when params.pulid_enabled is set),
  inserts the cross-attention call between transformer blocks at the
  intervals the PyTorch reference uses (every 2nd double block, every
  4th single block), and threads two new optional parameters
  (pulid_id, pulid_id_weight) through forward, forward_orig,
  forward_chroma_radiance, forward_flux_chroma, compute, and
  build_graph.
• src/stable-diffusion.cpp — loads pulid_*.safetensors via
  model_loader.init_from_file under the existing
  model.diffusion_model. prefix so PuLID-CA tensors bind to the new
  blocks naturally. PuLID-encoder keys (which live in the precompute
  tool, not in C++) are correctly identified as unknown. Adds
  load_pulid_id_embedding() to parse a small .pulidembd binary
  file and wraps its content as a sd::Tensor<float> passed via
  DiffusionParams.
• include/stable-diffusion.h — public API: sd_pulid_params_t
  (per-generation embedding path + weight), pulid_weights_path on
  sd_ctx_params_t, pulid_params on sd_img_gen_params_t.
• examples/common/common.{cpp,h} — three new CLI flags:
  --pulid-weights <path>, --pulid-id-embedding <path>, and
  --pulid-id-weight <float>.
• src/diffusion_model.hpp — extends DiffusionParams to carry the
  new identity embedding + weight; FluxModel::compute forwards both
  through.
• docs/pulid.md — usage, binary format spec, supported PuLID weight
  versions (v0.9.0 / v0.9.1; v1.1 deferred), memory budget notes, and
  a three-way SHA-256 falsification recipe.
• scripts/pulid_extract_id.py — reference precompute tool that
  produces the .pulidembd binary from a source portrait. Lives
  outside the C++ build because identity extraction (insightface +
  EVA-CLIP-L + IDFormer) is a heavy PyTorch stack that would be
  impractical to port to ggml just to run once per source person.

Why split extraction from injection

PuLID-Flux's identity extractor is a stack of three large PyTorch
models (ArcFace face detector + EVA-CLIP-L vision encoder + IDFormer
perceiver-resampler). Porting all three to C++/ggml would add ~5000
lines for code that runs once per source person and produces a 131 KB
output. By making sd.cpp consume a precomputed binary file, the C++
surface area is small (~600 lines), the heavy ML stack only needs to
run once per person on any backend that supports PyTorch, and adding
PuLID is decoupled from the active development on insightface /
EVA-CLIP / IDFormer.

Binary format

offset 0   : magic "PULIDV01"      (8 bytes ASCII)
offset 8   : num_tokens (uint32 LE)
offset 12  : token_dim (uint32 LE)
offset 16  : dtype (uint8): 0=fp16, 1=bf16, 2=fp32
offset 17  : reserved zeros        (15 bytes; header total = 32)
offset 32  : tokens, row-major LE

Typical (32, 2048, fp16) = 131 KB.

Verification

The three-way SHA-256 falsification recipe in docs/pulid.md
distinguishes "the feature is wired but doesn't do anything" from
"the feature is actively altering the diffusion trajectory":

Run	Expected hash relation
A: no --pulid-* flags	baseline
B: PuLID flags, --pulid-id-weight 0.0	byte-identical to A
C: PuLID flags, --pulid-id-weight 1.0	differs, preserves source identity

Verified on three backends with the same source code:

• Vulkan-AMD (RX 6700 XT, -DSD_VULKAN=ON): A == B byte-identical,
  A != C, C visually preserves source identity.
• Vulkan-NVIDIA (RTX 3060, same binary, --backend "diffusion=vulkan1"):
  A == B, A != C, C visually equivalent to the AMD output at the same
  seed (different bytes per the usual cross-backend nondeterminism).
• CUDA-NVIDIA (RTX 3060, separate -DSD_CUDA=ON -DCMAKE_CUDA_ARCHITECTURES=86
  build against CUDA 13.2): A == B byte-identical, A != C, C visually
  preserves source identity. PerceiverAttentionCA's pure-ggml graph
  code runs unchanged across all three backends -- no backend-specific
  conditionals were needed.

Per-image sampling times at 512x512 / 4 steps / Flux Schnell Q4 + PuLID:

Backend	Sampling (s)	Notes
AMD 6700 XT (Vulkan)	22	12 GB consumer card
NVIDIA 3060 (Vulkan)	11	same binary as AMD
NVIDIA 3060 (CUDA)	9.6	separate -DSD_CUDA=ON build

batch_count=3 was tested separately and confirms the long-lived-worker
amortization story: per-image sampling drops from 19.6 s (cold) to
~11 s (warm) as the model stays resident across batch iterations.

Tested with Flux Schnell Q4_K_S + PuLID v0.9.1 at 512x512 / 4 steps,
and Flux Dev Q4_K_S + PuLID v0.9.1 at 768x768 / 20 steps. 1024x1024 +
Dev + PuLID OOMs on a 12 GB card unless the VAE is routed to the CPU
backend via --backend "vae=cpu" (not just --vae-on-cpu, which only
offloads weights, not the compute buffer); this is existing
stable-diffusion.cpp behavior, not a PuLID-specific issue, but
documented in docs/pulid.md because PuLID users will hit it.

Tested with batch_count > 1 (verified each image gets the same
identity, different composition).

Not yet supported (called out in docs/pulid.md)

• PuLID v1.1 (pulid_v1.1.safetensors) -- has renamed key layout
  (id_adapter_attn_layers.* vs pulid_ca.*) and potentially
  different module structure. Follow-up PR.
• Multiple ID images fused into one embedding (the reference Python
  pipeline supports this; the current precompute tool accepts only
  one portrait per run).
• The --true-cfg negative-prompt branch -- PuLID only injects on the
  positive conditioning path in the reference implementation; this
  matches.

Backward compatibility

Non-PuLID generations are unaffected. The params.pulid_enabled flag
defaults to false and is only set when the model loader sees a
pulid_ca.* tensor in the loaded safetensors file. A regression run
of Flux Schnell Q4 without --pulid-* flags produces byte-identical
output to pre-patch.

File summary

include/stable-diffusion.h          +34 / -0
src/stable-diffusion.cpp           +120 / -0
src/diffusion_model.hpp              +5 / -1
src/flux.hpp                       +106 / -10
src/pulid.hpp                      +127 / -0   (new)
examples/common/common.h             +6 / -0
examples/common/common.cpp          +19 / -0
docs/pulid.md                      +220 / -0   (new)
scripts/pulid_extract_id.py        +135 / -0   (new)

Total ~770 added lines, ~10 changed. No removed functionality.